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. It sounds rather like an over-active moderator of multiple communities who upon banning someone from one of them also bans them from the others. Which might be a PTB, though depends heavily on the facts of each case, since it is exceedingly well-known that there are in fact trolls on Lemmy who consistently attempt to evade bans by making new accounts.
Anyway, if it is an over-active mod, then just block them and switch to post in other communities? Bam, problem solved.
The concern here I guess is whether that mod inappropriately abused their admin powers, thereby escalating the situation from one to multiple communities up to the instance level. Although I did not see sufficient evidence to confidently assert the outcome there one way or another, especially given how things such as lemvotes and the (paid iirc) Boost 3rd party app exists so any mod could in theory do the same... right?
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:-).