kbin by default throws everything that isn't formatted like a kbin thread into "microblogs". For whatever reason that thread on beehaw had an issue so it got put in the microblog section. I saw a similar issue result when one beehaw user tried to include a hashtag in the title of their thread, breaking it for both lemmy users and kbin users (lemmy displayed it weird, kbin put it in microblog section).
threads are essentially "special" microblogs with extra info. Mastodon posts will show up usually under the microblog section, but sometimes may end up in a thread depending on the situation.
On a technical level, lemmy users, kbin users, and mastodon users can all see the same content, just formatted differently.