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?

Asked differently,
what is it that allows lemmy/piefed/mbin to post to a community without explicitly tagging it, but doesn’t let hubzilla do the same even though it has a (relatively similar) notion of forum channels?
That’s a question for the Hubzilla devs. In Lemmy it’s achieved through a special field where you enter the community to post to.
I’m not super familiar with Hubzilla but maybe it is just that way because Fediverse integration was added later. And they probably based their integration off of Mastodon where it’s done through mentions in the post body.
I don’t think so.
First of all, Hubzilla was born into the Fediverse. In fact, the term “Fediverse” was coined in 2012, the very year that Red, that fork of a Friendica fork, was re-written against the new Zot protocol and to what would become Hubzilla in 2015.
Also, Hubzilla was the first Fediverse server application to support ActivityPub, two months before Mastodon.
The last thing that any Hubzilla developer, be it Mike Macgirvin himself, be it Mario Vavti and Harald Eilertsen, would do is take over any technology from Mastodon.
Unlike Mastodon and somehow like Friendica and Lemmy, Hubzilla has always had two kinds of mentions.
@is for mentioning channels/accounts the normal way.@!is both for direct messages and for posting to Friendica/Hubzilla/(streams)/Forte groups (the quick way; there are one or two more elegant ways).So normally, Hubzilla has three ways of starting threads in a group:
The first way is out of question. Lemmy doesn’t have a wall, and even if it did have one, it’d still require OpenWebAuth to recognise your Hubzilla login and let you post on the wall.
The third way is even more specifically Hubzilla. I’d be surprised if it worked with Friendica groups.
Another issue with the second and third way is that both result in either a DM or something that Fediverse software without a permission system understand as a DM. Lemmy communities, in stark contrast, expect public messages as start posts. I guess the devs wouldn’t consider it wise for Lemmy communities to forward DMs to all their members. Even though this is literally what groups on Friendica and all its descendants have been doing since 2010.
Also, Lemmy doesn’t know
@!, but it does know what to do with the universal@. Mind you, neither Hubzilla nor Mastodon nor most of the rest of the Fediverse knows!. (Hubzilla was originally supposed to support both@and!like Lemmy plus an additional!after either for “private”, it even ended up in the user documentation, but it would have been overkill and utterly impractial to actually implement, seeing as Hubzilla forums need DMs to work anyway.)And so the only way to start a new thread in a Lemmy community from Hubzilla is with a public post that all your contacts receive, and that mentions the Lemmy community with
@.On a sidenote: We’ve discovered today that it’s entirely impossible to start threads in an Mbin magazine from Hubzilla. Mbin doesn’t even accept
@mentions from Hubzilla.I would’ve thought formatting quirks like
@and@!only need to be recognized by the poster’s instance, where it would then be converted into a generic markdown link as well as be added to the"to"or"cc"field of the ActivityStream?