• I think this is an oversimplification. ActivityPub is the server-to-server protocol. Mastodon and Lemmy each have different client protocols. I think the ! search syntax is part of the Lemmy client protocol specifically, and won’t work on Mastodon.

    Lemmy communities will appear on Mastodon as though they are users. So you can find the programming community on Mastodon by searching for @programming@beehaw.org. If you @mention that account your post becomes a thread on Lemmy. You can follow it, but beware that you will get all replies in your Mastodon feed, not just top-level threads.