I've been doing some more exploring, and at this point in time, it seems to be Friendica specific. It has group/forum/community functionality built it, and recognises the ! notation for groups.

It's a bit finicky from the Friendica side, as it won't work if you haven't already subscribed to the group via the Friendica interface, but after that, it's pretty simple.

I'm assuming it's also relatively recent functionality on the lemmy side, as my instance isn't completely up to date, and I can't post to it from Friendica, but I can post to lemmy.ml.