In the federated AP data, threads are of type Page and microblog posts are of type Note.

The usual threadiverse model is Page for the thread and Note for comments. On Mastodon, it's Note for everything.

I think (my conjecture) the reason why Ernest (kbin's dev) probably originally implemented Microblog posts as a separate feature (despite them still being community-bound like threads) is because Page is being used in an unorthodox manner (a Page is a web page, not a post, by spec), and there's no guarantee that other projects will support it as a post type. It's better to provide support from your side than rely on the other side offering support.

How is that possible?

on mbin I'm noticing microblogs can be posted to a magazine

Mastodon users can post microblogs to magazines too, by mentioning them. "Posting to a magazine" literally just means the magazine is part of the post's audience. Same with communities on Lemmy.

Similar to email, each post has a list of recipients.
This is the one of the first microblog post I see in this magazine:

"to": [
    "https://gehirneimer.de/m/mbinmeta",
    "https://www.w3.org/ns/activitystreams#Public",
    "https://gehirneimer.de/u/BentiGorlich",
    "https://kbin.melroy.org/u/melroy",
    "https://fedia.io/u/blued_gear",
    "https://kbin.earth/u/jwr1"
  ],
  "cc": [
    "https://kbin.earth/u/Maeve/followers"
  ]

Notice how the first item is the magazine itself. The Public entry means that the post should be publically visible on instances that receive it. And then are direct mentions made in the post listed. Finally the author's follower list is linked. The magazine sends it to its subscribers when it receives it.

Any post sent to the magazine which contains those first two entries should become visible on the magazine. Provided it doesn't fail any checks.

and I've even seen microblogs from Lemmy users.

That's probably due to bugs on either side. Most likely a comment (since those are Note, like microblog posts) which Mbin somehow couldn't interpret as such, and so it fell back to treating it as a microblog post. I'd report such cases to the devs.

If you meant you saw Lemmy posts on Mastodon, Mastodon does understand Lemmy's use of Page. So if someone on a Mastodon instance follows a Lemmy community, than that instance will receive Lemmy threads posted to it.

Also, if you can post a microblog to a magazine, what separates it from a thread?

For microblog posts:
Cons: no downvoting (maybe a pro? subjective), no title, can't make the post itself a link to something Pros: wider compatibility by not relying on receiving instances to know that a web page is somehow representing a post

Also, you'll annoy Lemmy users by making microblog posts, that's another pro (I'm joking, it's not).

Honestly the only real difference is that a thread has a title and can have a linked URL. Otherwise they are pretty similar to a user. In a technical way, a thread is the native lemmy post and a microblog is the native mastodon post

@srasmus@fedia.io additionally to what BentiGorlich said, there's also different compatibilities depending on the type of post you send out.

Threads are received normally on Lemmy, PieFed and Friendica. If you added tags to them, they don't appear on Lemmy but do on the other two.

Threads received by microblog platforms, I am only familiar with two cases, Mastodon and Misskey. On Mastodon, they are received the same as Lemmy posts, title, link to the original post and if included in the OP, the embedded image. On Misskey, they appear as long microblog posts. And in both cases, included tags are displayed.

Microblogs get a bit wilder.

Hashtags must be included in the post body, instead of having a field for that.

If you send them to a community, or mention the community as an user, they appear in that given community. Some times they don't on Lemmy instances though so you may need to manually search using their links on an external instance while logged in there, and the post appears only in the instances you search it on from my experience.

Also on Lemmy specifically, the first paragraph is used as the title, so avoiding links and hashtags in that paragraph may be recommended.

On Mastodon, Misskey, Friendica and PieFed, they get received as long microblogs. Also though boosts are available for all posts and comments, if you boost, it only gets received by boost-compatible platforms.

Unlike threads, there's no dislike button in microblogs (though on Lemmy and Friendica at least that's still possible).

If you follow @bsky.brid.gy@bsky.brid.gy, your microblog posts also get bridged to Bluesky and other AT Protocol platforms. But threads aren't federated. Also do mind bridged posts only display the first 300 characters before linking to the original post, meaning if posting to Bluesky and Lemmy at the same time from an Mbin instance, hashtags and the headline must fit within those.