While strictly true (calories being a measure of energy and valid for both heat and fuel potential), the difference is - unsurprisingly - small :)

Eating a large pizza (1200 kcal) straight from the freezer instead of heating it, gives you roughly 25 less kcal.

Well duh. If it’s in the freezer that means you haven’t eaten it!

So I found this website that lists specific heat capacities for various foods, and while it doesn’t list “snacks”, dry foods values seem to range from 0.3 to 1 cal•g^-1•K^-1 = 0.0003 to 0.001 Cal•g^-1•K^-1. Assuming no phase change (i.e., melting) and otherwise temperature-invariant heat capacity, the energy required for heating a 100 g snack from freezer temps (-18 °C) to body temp (37 °C) is 1.65 to 5.5 Cal. More realistically, we can compare to eating an ambient-temp (20 °C) snack; that difference is only 1.1 to 3.8 Cal… in either case, the difference is negligible, generally < 1% of the calorie count of the snack itself.