Are you absolutely certain of that - that is not what the feature description seems to indicate?
Piefed can't control lemmy instances like that. I don't know if it crosses over Piefed instances (I'll ask rimu later) but I can't see why it would work on lemmy instances. Similarly lemmy.blahaj blocks all downvotes, but it can't stop people outside of lemmy.blahaj from downvoting posts from lemmy.blahaj. People still see downvotes on lemmy.blahaj communities if they're looking at them from lemmy.world. It just means that from a lemmy.blahaj user's perspective, there are no downvotes on anything.
I would have naively guessed that it would be trivially easy for PieFed to know which users were subscribed to a community vs. not, but maybe it's in an inaccessible form or something.
Yeah, Piefed can read this - but it can only control the downvotes on its own instance.