This assumes a lot of things, but mainly:

1. Biochemical properties of protein, are identical to that of fat, and identical to carbohydrates as well.

I.e. If my truck runs well on unleaded, grade 97 fuel with X amount of energy, I can simply replace it it with any other fule (diesel) for equivelant energy amount. Different properties of fuels don't matter

2. Different bodily functions and enzymes behave in the same way toward any food source, regardless of their properties.

3. There is no such thing as Insulin. Or it's there, but it doesn't matter in terms of sugar metabolism. Same can be assumed for glucagon.

So, if the above assumptions are correct, then you are correct, it's just about calories!

Wa'il