Here's a community on a mbin instance with people from piefed posting into it - https://piefed.social/c/firefox@fedia.io

It happens.

fedia.io is the only busy Mbin instance, so focussing on that will bring up some posts. See https://piefed.social/instance/fedia.io/posts and https://piefed.social/instance/fedia.io/people

(sorry mate, as always, I continue to be a complete moron upon this stuff)

I’ve never noticed a Lemmy or Piefed account post to a mbin community

AFAIK tardigram is mbin and their magazines look here like just another community
!ciencia@tardigram.com for example

Hmm, it could be there are two different types of mbin "posts". I just saw a mbin post asking about the difference between threads and microblogs: https://threadiverse.link/fedia.io/m/AskMbin/t/4407634

I vaguely recall Rimu talking about wanting Piefed to be interoperable with all the other major federated services, with the developers focusing on Mastodon currently.

I suspect Rimu has talked about this before, possibly with greater federation as a future feature, I just haven't found where yet...

Regarding mbin, I've seen comments/posts federate both ways in the past (seen mbin posts and comments, had mbin users respond to piefed and lemmy posts and comments). There aren't very many active servers (currently 16) so that may be your issue. There are a list of the active servers in this post: https://piefed.zip/c/fedilytics@piefed.zip/p/1863298/growth-of-the-instances-20260929 .

Although, I've never noticed a Lemmy or Piefed account post to a mbin community. I wasn't looking for it though, so could work, just never paid attention to that aspect.