Yes, this is certainly a possibility, I agree. But this post was about "privacy" (look at the title). And there I don't see anything concerning. Talking about "adding features on top": they can do this even without federating (because of infinite money and human resources). I think the reason they came up with idea joining fediverse is to get as bigger userbase as possible to compete with Twitter(might be other reasons, but idk). I already tested out their app, it's really convenient for Instagram users, to move there. But their search capabilities are even worse then in Mastodon, they can only search for other accounts. No post search, no hashtags search etc. So it's kinda worse in technical aspects then rest of fediverse.

In conclusion I think their main goal is to compete with Twitter, not the fediverse. My opinion is to let them fedirate, and if they start doing some bs, block them.