This is my understanding as well, @a@kbin.social¹. And with this understanding, I concur with @anonymouse:
they made things overly complicated
Indeed. It’s not a meaningful separation particularly when the microblogs have no significant enforcement of being “micro”. The /feeling/ I get is that kbin threads (which are very poorly named as @s6original discovered) are for high effort often lengthy content with a graphic, formatting, and more thought-out content. But kbin supports formatting in both which makes them redundant.
I suspect if someone makes a lot of short off-the-cuff “threads” a moderator might want to have a word with them.