These hips don't lie : you got cancer

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).

Technically it measures how much you can heat up a known volume of water if you burn the food. We have no way of measuring how much of that energy released by combustion actually gets absorbed and translated to ATP in the body, but it’s the best estimation we have of the relative energy content of foods.

There’s some carbohydrates, proteins, and fats that our bodies don’t seem to convert to energy (or only partially convert) but still technically contain “calories” because they’re combustible. Sugar alcohols, fiber, etc.

Plutonium doesn’t combust, but it would heat up water in a calorimeter. Really the test method’s applicability kind of falls apart when you start testing undigestible materials.

It looks like the underside of a microfiber towel