Let's remember that Fournier is an unrestricted FA and he spent just a few months on the Celtics, during which time the team was mostly non-competitive and he had a very bad case of COVID.
It's not exactly shocking if he wasn't especially interested in staying.
Also, if the Celts don't re-sign him that might be because they're cheap ... or it might be that they're not interested in tying up cap space. They haven't extended Smart or Rob Williams yet, either.