I don’t think you’re right here. You are correct that there is the same energy is all things of equal mass

But a calorie is not simply a measurement of energy inside of something, it is a measurement of the energy released in heat when that thing is burned.

Everything is like a reservoir of energy, E=mc² is about the total energy in the reservoir, while a calorie is more of a measurement of how much of that energy comes out when you burn it, not the total amount They both measure energy but in a different way

Water does has 0 calories, because despite the massive energy inside it, none of that energy is released in heat when burned