Adam Mosseri:
Second, threads posted by me and a few members of the Threads team will be available on other fediverse platforms like Mastodon starting this week. This test is a small but meaningful step towards making Threads interoperable with other apps using ActivityPub — we’re committed to doing this so that people can find community and engage with the content most relevant to them, no matter what app they use.
I wouldn’t call that a good reason to team up with Meta, but I would call it a plausible. Everyone does not want to federate with the largest social media company in the world, I can promise you that. If you like federation, you’d probably like it to not be engulfed by megacorps (unless you stand to profit from it).