The fact that the different platforms call things differently for similar content is confusing for sure. Mastodon sends all its content as a single “type” under the hood known as a “note.” Mastodon doesn’t call them “notes” though. It calls them “posts” or until a little bit ago “toots.”
Those “notes” show up on other platforms where different terms are sometimes used. Misskey doesn’t call them “posts” but “notes” for example. Kbin called them “posts” and they show up in the microblog section of a magazine.
To answer your question about this thread:
This Thread was created as an "Article" or "Thread" in newer code commits. (That’s confusing for sure) and it isn’t sent under the hood as a “Note.” It’s sent as something called a “Page” which different platforms will treat differently. Some platforms use and understand “Pages” in a consistent way while others don’t.
As for the question is this both a Thread and a Post touches on which platforms’ interpretation of those terms.
Part of the confusion comes from the fact that the underlying format of the data exchanged between instances and platforms (ActivityPub) is actually pretty vague about “meanings” of the different types. Every platform is more-or-less free to define what the different “types” mean (Notes, Pages, and a bunch of others) within the vague guidelines.
So when one platform starts to gain a large number of users, the interpretation of what a “Note” for example, will tend towards that larger platform’s meanings if other platforms want to have a more “consistent” experience.
The story of why the meanings are vague is an interesting one and I’m not knowledgeable enough to explain details but from what I understand, it’s a result of a lot of people trying to come up with a universal standard for interacting between platforms that are going to approach things very differently all without a central decision-maker driving it.
If instead of Mastodon being the largest platform on the Fediverse it has been a Reddit-like-alternative like Lemmy or Kbin, the interpretation for the various types of data would likely be different today.
To partially summarize
Colloquial term vs ActivityPub object
Mastodon:
Post = Note
Kbin:
Thread = Page (at least for articles. I still have to check the others)
Post (microblog) = Note