You're not wrong. And this isn't bulletproof. But small n studies are often needed on a particular subject in it's academic infancy almost like a proof of concept. A meta analysis like this helps expand or elucidate things that small n studies can't. They can all then be used to pitch for funding for the larger, more representative, more comprehensive studies.

In that framework, I see it as still very useful science, even if it's not entirely settled.