I’ll see what I can find. I am not a developer on the Hubzilla project, but I’ll try to figure it out. Maybe @macgirvin and @Mario can shed better light on how all this works on the back end.
One thing that frustrates me is that Hubzilla had to format its ActivityPub messages in a way that is compatible with Mastodon. And since Mastodon does not currently understand what a forum or group is, a lot of creative hacks had to be made to get forums to work with Mastodon. (As someone developing themes for Hubzilla, this affects me since it changes how the data is stored in the database.)
For example, since Mastodon will reject messages from people you don’t follow, the first post in a forum thread must be owned by the forum, and not the actual author. If it is not done this way, Mastodon users who follow the forum will not see new posts.
This may be changing as Mastodon is considering adding groups. This would mean that forums on Lemmy and Hubzilla don’t have to pretend to be a regular social media account to work with Mastodon.