Ndtyhij

it is possible to have half a calorie, or 4.582394 calories

It's also possible to have half a cow or 4.582394 cows. Indeed, the same is true for almost every countable noun that existed at the time when "fewer" came to be the word we used with countable nouns. Perhaps it's impossible to have half a thunderclap...

This shows that it's a mistake to think that the existence of fractional quantities means that a noun is uncountable. Rather, countable nouns are those that can be modified by a number and which have singular and plural forms.

While calories are continuous to scientists, most lay people don't think of them that way. Food and activity calories are always reported in whole numbers, often only precise to hundreds or thousands, because for most people's purposes any more accuracy is not meaningful or useful.

So in common use we treat them as discrete units, and the language we use reflects that. We use "more" and "fewer" when comparing things, and ask things like "How many calories does an egg have?" and "How many calories do does an hour of exercise use?" (rather than "How much", which would be used for non-countable quantities).

You shouldn't use "fewer" instead of "less" necessarily. Look at this NGram:

Link

In many cases "less calories" is more common than "fewer calories". In no case is there an enormous difference either way.

It depends on how conceptually you see what it is you're describing with either "less" or "fewer". In this case it's whether you see the calories as individual units (plural), or one sum of things, ie., energy (singular). Oxford Online Dictionary blog explains:

(In a supermarket/store) Ten items or less.(Correct or incorrect?)

Firstly, having absorbed the guidelines above, you may suppose that
some supermarkets are grammatically on the ball by displaying notices
at checkouts that state ’10 items or fewer’ (fewer rather than less
being the right choice because it’s referring to items, that is, a
number of things?). In fact, there were reports a few years back that
Tesco had replaced their signs reading ’10 items or less’ with ones
which said ‘Up to 10 items’, so as to placate the sticklers. Sorry, no
need! This is an example of hypercorrection. Pocket Fowler’s Modern
English Usage puts it very succinctly:

• ‘Supermarket checkouts are correct when the signs they display read 5
  items or less (which refers to a total amount), and are misguidedly
  pedantic when they read 5 items or fewer (which emphasizes
  individuality, surely not the intention).’
  Link
  

The point is fewer is used for plural and less for singular. But the mental notion of whether something is singular or plural seems to be an influencing factor in whether "less" or "fewer" is used.

Look at the difference in usage between:

• less than two miles
  
  and
  
  


• fewer than two miles
  
  

The full explanation of "fewer" and "less" is not just as simple as the explanation in the answer you linked.

"Fewer" refers to quantity in general, even when the notion of units is ambiguous or abstract. It can refer to discrete, continuous, and even infinite quantities.

There are fewer points in the Cantor Set than on the closed interval [0 ... 1]. Note that both sets of points are infinite, but the Cantor Set is a lesser infinity.

There are fewer kilometers to travel from Earth to the Moon than from Earth to Mars on any minimal-energy trajectory. If the journey is powered by conventional rocket propulsion systems, the engines will burn fewer calories, and proportionally fewer Watts. Also note that at the end of that last sentence, Grammarly suggested I use "fewer."

There are fewer odd numbers than integers in the set of whole numbers ranging sequentially from 1 to any fixed positive integer N greater than 1.

Why? Because this is English.

I think the citation you provide is misleading because it refers to the intrinsic qualities of the thing described by the noun.

My suggestion: Don't focus on whether a noun refers to something divisible. (Virtually everything is divisible.) Instead, just research to find out if a word is "countable" or "uncountable".

Sometimes, words will function both ways -- three beers are countable, while "some" beer is not countable. If you say, "I would like a lot of beer," the listener might bring you a single large keg of beer, but if you say, "I would like a lot of beers," the listener will bring you many cans or bottles.

In short, don't think too hard about it, just look up the word you have a question about in the English learners' dictionary:
https://www.ldoceonline.com/dictionary/beer

Both less and fewer are correct, however, many English speakers prefer fewer in this context, especially in formal speech. Here's the relevant portion from the Cambridge Grammar of the English Language, Chapter 13, Section 4.1.2:

The relation between less and fewer is fairly complex. In non-count singulars only less is possible: Kim has less/*fewer money than Pat. In plural NPs we have:

1. 
   She left less than ten minutes ago.


2. Less/Fewer than thirty of the students had voted.


3. He made no less/fewer than fifteen mistakes.


4. You pass if you make ten mistakes or less/?fewer.


5. He took less/*fewer pains to convince us than I'd expected.


6. He made fewer/less mistakes than the others.

Both [1] and [2] have than + numeral. In [1] ten minutes expresses an amount of time rather than a number of individuated units, and in such cases fewer is virtually impossible – just as few would be in a comparison of equality: She left as little/*few as ten minutes ago. Similarly with We paid less than thirty dollars for it; She's less than forty years old; We were going at less than ten miles an hour. In [2] we are concerned with countable individuals and little cannot be used in a comparison of equality (*as little as thirty of the students); nevertheless, for inequality less is more common than fewer in this construction. The same applies with percentages: Less/Fewer than 30% of the students had voted. Construction [3] has the comparative form following no: though the interpretation is count plural, less is here again more common than fewer. Construction [4] has or after a numeral: less is the usual form here, with fewer quite marginal; this construction is widely seen in supermarkets, with the fast checkout labelled eight items or less, or the like. In [5] pains is plural but non-count rather than count (we can’t ask how many pains he took), and here only less is possible. Finally in [6] the comparative occurs directly with a count plural noun: both forms are found, but less is subject to quite strong prescriptive disapproval, so that fewer is widely preferred in formal style, and by many speakers in informal style too. [emphasis mine]

[Quotation edited for formatting/readability.]

So the short answer is that the traditional rule is an oversimplification, but it has influenced English speakers to prefer fewer over less in this context. But nevertheless, both forms are found and are thus not wrong.

We simply don't speak of 5 Energy.

The key difference here, mathematically, is rationals vs real numbers. That is, even if you cut your kilocalories in half, you would always have a finite-length number to describe the amount. Hence, the rationals as well as the natural numbers are called countable. The set of Real Numbers on the other hand is an uncountable set, because for every finite-length number, you can have an infinite amount of infinite-length real numbers that start with the finite length number. The details are tricky, I'm not a maths teacher.

So far so good. Why then is energy uncountable? It's a historically grown thing. The whys and hows of this fact should be part of a different question, though. (I mean, I really don't know).

Suffice to say that the distinction between the measured object and the measurement result is purposeful.

In expressions like "less than ten minutes", "less" is correct because it could be considered as referring to the implied word/concept "time", as in "less [time] than ten minutes". The emphasis is not in the units as the exact quantity of time in non-integer (in general) minutes is not important, as long as it is less than ten minutes. So for continuous quantities, as energy, time, and distance, I consider both "less" and "fewer" to be technically correct, with "less" being better. When someone refers to something discrete, as in "ten free throws or less", I would consider "less" to be technically wrong but it still sounds good to me. Just as when adjectives are used instead of adverbs.