I think single account ActivityPub implementations are addressing a weakness of the Fediverse: one's identity (handle, username) is tied to an instance they have no control over. If that instance shuts down users lose everything. With a single account instance, you take that control back. And since it doesn't need to scale the architecture can be much simpler and can be deployed to much cheaper infrastructure.
The demo was not straightforward, though. And I didn't quite get how a user can follow Mastodon users, for example.