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.
That doesn’t really apply to Lemmy’s content though, since unlike Lemmy.world, Threads users won’t be able to create /c/ communities. If a Threads user wants to post to a community in a way that Lemmy recognizes them, they’ll have to post it to one under a Lemmy instance’s control, or Lemmy users won’t see a thing.
That was just an analogy, I believe Threads is targeting federation with Mastodon rather than Lemmy