I'd been under the impression that Hubzilla runs on ActivityPub, but evidently that isn't quite correct. As I understand it, Hubzilla runs on a protocol called Zot.
If you want to federate with accounts on ActivityPub, you need to install the ActivityPub app on your Hubzilla account.
This was fairly straightforward to do, but realizing that this was necessary was not straightforward.
Binder