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.
While that’s true, I’m not sure how many people are using Mastodon that way and if that’s actually the main concern. In the end it still is meant to be a public platform. Not on the same level of “private messages and photos” where most people would probably be very concerned.