I don't see why they would bother with the fediverse as it exists to be honest. To me it seems like a liability from their point of view. Not sure if they've spoken more about this but Facebook getting in more shit by having their users exposed to stuff that they don't explicitly control doesn't seem like something they'd want.
That being said, I feel like defederating with them if needed is a solid idea but their sheer size may make that decision difficult for instances that are looking to grow given that they've already amassed twice the accounts of the Lemmy fediverse in a few hours. Now not all growth is good growth like you've mentioned but there's no partial defederation so either you leech on some of their userbase or you don't.
I see some places going for growth if that's an option which may not necessarily be a bad choice (unless they impose strict rules to follow if you want to federate with them) given that facebook has the capital to bury us with if they choose to so our compliance probably won't have a very big impact on how things play out in the long run.