The only correction I would make is that Kbin does let you follow communities from other instances (even Lemmy instances).

Until a couple weeks ago, it would show local magazines first… but if you dug deep enough, after all the local magazines were listed it would start listing remote communities across Lemmy/Kbin.

It’s changed very recently to always sort everything by subscriber count, with an option to toggle between local magazines and everything on the threadiverse.

As far as “I tried to go to posts I knew existed but weren’t showing up” - like everywhere else on the fediverse, someone needs to follow that content first. So the reason why they saw their Mastodon content is because someone followed their Mastodon account from Kbin. When they searched for things that didn’t appear, it’s because nobody on Kbin was following those accounts.

If they searched the full @username@instance.social and hit “follow”, then future posts would appear in the microblog tag and be searchable.