I've been under the impression people started using the term threadiverse to describe the Lemmy/Kbin side of the fediverse because we exist in Reddit style threads and interaction with microblog style fediverse posts is obtuse at best. We're practically in a separate bubble over here, and that was the cause of the new term.

Edit: The first time I saw the term used was when FediDB made a page for tracking Lemmy+Kbin users

Edit 2: Archive.org link to the Threadiverse page from June 15th, half a month before the Threads name leaked.