So I think it's the PubSubHubbub portion that does what I'm looking for.

Current understanding: I make a post & my instance pushes that post to each instance where someone follows me.

Which means if some single-user instance's user follows me, but no one on my instance (including me) follows them, then my instance is "aware" of theirs, but not in a way that their posts would show up on our federated timeline.

Anybody poke holes in that?