Activity Pub has a few parts, Lemmy implements the Threaded message part.

Mastodon implements a short messaging (posts) part. Meta’s Threads will implement this.

KBin implemented both parts, within KBin you’ll see microblog as an option for magazines (communities/subredits). This shows either ‘posts’ made to the magazine or posts with hastags associated with the magazine.

The posts and threaded message parts have overlap in how they work so mastodon users can see certain threaded messages and comment on them.