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.
If they thought the fediverse was insignificant they wouldn’t even bother. Them spending publicly visible time and energy on it means there’s something for them to gain.
But yeah, I’m not entirely sure what yet. Like you said, reading data is freely available, so it’s not just that.