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’m sure they have a plan (otherwise they wouldn’t be doing it!), maybe it relies on using their app which also has your real name and phone number, maybe it’s for some legal loophole which means all fediverse users technically agree to their terms just by federating. I don’t know what they’re up to, but given their previous behaviour I think it’s safer not to even let them try!