They didn’t say the presidents were criminals, they said they were war criminals, which does have a specific legal definition based on specific laws. Ones that many world leaders, including us presidents, abuse on a regular basis sure, but whether someone qualifies as a war criminal specifically is very much a legal matter, not just a moral one. That said, you don’t have to be an expert to know laws or accuse people of breaking them, you just need to be an expert to navigate the legal system effectively, which in an ideal world is intended to ensure that the laws are upheld fairly. Unfortunately we don’t live in an ideal world.