@Eigengrau

Both kbin and lemmy inherently support microblog posts because that's what threads are behind the scenes. Kbin and Lemmy just choose to display things in a more reddit-like manner. The microblog tab on kbin is for communication with the wider fediverse, but lemmy.world asking kbin.social for new threads made by its users will result in microblog posts made from kbin.social showing up as normal threads on lemmy.world. Because threads and posts are pretty much the same behind the scenes.
What lemmy shouldn't be able to see is mastodon posts. I'm not sure though if kbin replies to mastodon posts make those appear on lemmy, that's a possibility but I don't know the answer there.

@Eigengrau
Out of curiosity, our Magazine owner tested https://kbin.social/m/arknights/p/911556 . Which is why guide to post to Lemmy from mastodon works , while for Kbin it's sorted to Microblog.

It's the fediverse, of course we can. KBin just provides a better UI for that.

However they don't really pop up in any feed automatically...

Are you sure you didn't reply to a lemmy post/comment through the microblogging UI? Microblog users can create lemmy posts by tagging the community, and can respond to comments etc.

@Eigengrau as far as I know they don't; Mastodon-type users can see Lemmy threads and users and reply to them, but Lemmy users can't do the reverse with microblog posts.