My very rough understanding is that ActivityPub is strictly about the protocol of the content and nothing to do with how it's handled past showing it. So accounts and interfaces aren't even part of it. Now, could a subset of it, or a parallel protocol for account handling be done (or maybe it is?) It would indeed be nice to have even more feel of a singular "system" but still the pluses of federated across various methods. It might also help fight against some of the bots/spammers who use the many-faced instances to do their stuff making it harder to pin them down.