I think the portmanteau "threadiverse" works for this situation and it's what I've been using to refer to anything in the general Lemmy/kbin side of the fediverse, but I think people just referring to the platform is inevitable. People talk about posts "on Mastodon" even though there's like 15 different services you can use to post to the blogging side of the fediverse, like Pleroma, pump.io, etc. It's worth thinking about in situations where you're like "Hi Lemmy" because you're definitely talking to more than just Lemmy, but any time you're talking about your personal experience of where you saw a thread I think it's perfectly accurate to use the name of that platform rather than having to say you saw it "on the fediverse".