Alright, so I can start a thread/create a post on lemmy from hubzilla, but the behaviour’s… weird?
Hubzilla has a notion of channels that can be individual users or forum-based communities. You can share a post with public, some other fine-tuned lists, or post them on forums.
Lemmy communities correctly get recognised as forum channels. Why is it then that I still need to create a public post and tag the community channel to create a post rather than just posting and sharing with the forum channel??
TL;DR: Hubzilla forum channels = lemmy communities. Why still make public post and tag community then?

In ActivityPub you are only supposed to be able see the activities from the
peopleactors you’re following. In order to tell an ActivityPub server like lemmy that something should also be viewable by the public it needs to be addressed to the special public group"https://www.w3.org/ns/activitystreams#Public".When a lemmy user makes a post, their instance can’t just send the post to everyone in that community, because the other members are only following the community, not every other individual member in it. So instead they will simply address the post to the community-actor, and that actor will then announce/boost that post to all the members.
I think in lemmy posts are also generally addressed to the public to make them viewable by non-members in the Local/All feeds. So when a lemmy community-actor is receiving a post, I’m guessing that it requires that post to already be addressed to the public, because otherwise it might be publicizing what was supposed to be a private post.
Ah, that makes sense.
I still wonder if there’s a way to start a lemmy post from hubzilla without explicitly tagging the community, since it kind of seems like a nuisance on Lemmy’s side and is exactly the kind of behaviour mastodon users interacting with lemmy get flamed for.
I guess this is the best I can do though 😅
I’m not familiar with Hubzilla, but it would seem fairly trivial to make it possible to address channels explicitly in a Multiselect field somewhere, rather than it being implicit by mentioning their handle in the body. In the case of Mastodon it’s just the UX they want to go with I think