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.
because if that was going to happen it would have happened the last time the migration from twitter to mastodon happened and then mastodon users got all angry when twitter users suggested adding things from twitter like quote posts, etc (which threads now has).