@davew Oh neat! I hadn't seen that. 🙂 Yeah, at that point you have something very close to ActivityPub.
From a first glance, one difference seems to be that with AP your follower list is based on individual accounts and not servers/aggregators. Multiple accounts can have shared inboxes to reduce redundant traffic, but they're not mandatory. So arguably AP gives you more knowledge of who your followers are. Now, whether that's on the whole a good thing? 🤷