That’s… highly odd, and unusual behavior. But even as you described it, I disagree then that it is an “instance ban”, if you can still post to Discuss.Online after being banned from those communities.
That's the thing. If you're banned from an instance, you can still post to communities on that instance - they just won't federate out. Only community bans hard-lock a user from being able to post to a community.
I went through 29 comments you made in the provided link - we really need an “expand all” or a “search through comments to this post” functionality:-).
I'll quote myself:
As far as I know, if someone from lemmy.world is banned on lemmy.zip (an example) - they are only directly banned from lemmy.zip communities they have commented on. This is why the modlog shows users simultaneously banned from like a dozen comms at once from time-to-time - they’ve just been banned from an instance
But any communities on lemmy.zip that the user banned from lemmy.zip hasn’t commented on, they can still post there. It just doesn’t federate out.
That means a user from lemmy.world could be a nuisance on a lemmy.zip community without the local lemmy.zip mods noticing. This absolutely happens on piefed bans of lemmy users now, and I assume the same in reverse. I saw a user banned from piefed.social still posting on my television@piefed.social community from lemmy.world, but it just doesn’t federate out.