this is a commonly repeated falsehood obvious joke

And, if I have to explain the joke: it's just E=mc² (the Einstein thing ... well, the Einstein's thing's approximation), the energy (E) is the same for all mass (m) since the c is a constant.
You get the same 21 billon kcal from 1g of apples as from 1g of plutonium.
And since it's usually well known humans do not devour mass into pure energy that might trigger ppls sense of humour.
(Additionally the idea of eating metal to seek nutrition might be funny, but we do need some metals \m/.)

Also "potential energy" phrasing is weird in that context.

There are 2 different definitions of calorie.
This "fun fact" mixes up the two definitions

It's not even two definitions, the kcal is absolutely the same, it's just used to measure two different things (mass energy vs the sum of what an average human can extract via chemical processes). I see you def understand that, but it's not a different definition of a calorie (in the same way as length vs width of an object isn't a different definition of a metre).