@RheaSilvia thats because cooking at its core is IRL chemistry or in a fantasy setting alchemy.
You're combining organic and inorganic materials in such a way that they are transformed from their original state. The application or removal heat also furthers changes the materials. Depending on how it's made, food can be filling but lack nutrtion, nutrient dense, medicine or poison, and any variation of others.
And given what was said in chapter 1, all his skills are the result of cooking; legendary level sword skills from prepping ingredients, alchemy from the act of cooking itself, metalurgy from working cooking utensils over a fire, but that might be a mistranlation, cause wtf does metalurgy have to do with assassination.