instance banned person was able to comment on a post to a community in that instance?

Oh yeah, this is expected behavior. A bans U on instance B without a community ban. U can still post and vote on their instance B, but none of that content will federate to A.

However, other instances C D E wont see any of U's content, because A isn't forwarding it... unless U is responding to content from a user on a different instance, then B will federate the content to the content creators instance (which doesn't drop it) and A which will drop it.

Basically, what this means is things get kinda weird when a user is only instance banned but not community banned.