I think many users of reddit did not have a good opinion of Meta prior to any Reddit migration. And despite the dislike for Reddit now it is still a site where using it you know you are not going to be directly exposed to Meta or in direct partnership with them. At best it's people reuploading Meta content but not direct access to Meta services. And I don't think Meta is forcing Reddit to sign NDAs to talk to them either.

So Reddit for all its criticisms is still a Meta free place. So leaving Reddit then running into the hands of a partnered Meta instance can be an off putting prospect. If people had to choose between using a Meta affiliated instance or reddit I think many would prefer to go back to reddit.

I think people against it want more degrees of separation from Meta where separate accounts are needed to utilize their services. It's not like there isn't over a decade of Meta's track record to look at or even more recent examples like theit app requirements and permissions to use threads that raises red flags and makes people not want to use them even if it is indirectly through a fediverse alternative.