ActivityPub integration is another feature they can use to get attention.
See, that's what I don't understand. ActivityPub means nothing to the vast majority of potential Threads users. There's no way that Meta is going to use ActivityPub to gain users; all they have to do is what they HAVE done, leverage Instagram. The only thing that makes sense to me is that they may be hoping that federation will allow them to get around the EU's limitations.
But even that doesn't really make sense. Zuck doesn't really care that much about regulations. He breaks them all the time. Which leaves me with the question, why ActivityPub? What aren't we seeing?