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.
Well a good reason could be that it brings federation to the masses. You know, like everyone who uses federated networks wants it to be. This isn’t some exclusive club and wider adoption is a good thing.
If only to prove that it can work.
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).