R Daniel Braude (Learn Shabbos, pp. 120ff) writes that it is only cooking if using a heat source and if it is the normal way of cooking. Since "pressure-heating" water doesn't have a heat source and is not the normal way to cook, it wouldn't be prohibited.
Specifically, he writes
- The Torah prohibition of cooking is transgressed by using fire and those things that derive their heat from fire because that is the
normal way to cook. However any heat source that gets red-hot is also
- Cooking under the sun is permitted, as this kind of cooking is not normal and therefore not called cooking in a Halachic sense (but
Chazal prohibited cooking using anything that derives its heat from the sun as it is very similar to something that derives its heat from
- A microwave could theoretically be permitted (ignoring electricity for the moment) as it does not actually have a heat source. However it
is prohibited since it is (now) the normal way to cook
- The prohibition of cooking only applies when the heat is capable of causing the food to become yad soledes bo
I checked this with R Binyamin Tabady but of course, consult your rabbi
before implementing anything you learn here.