Let's clarify something here. Mastodon follower only posts don't have the "public pseudo user" addressed, do they?

That's the important piece that this whole thing hinges upon

If it is present, Mastodon is a fault. If not, Pixelfed messed up.

Am I mistaken?

For example, a followers-only post is addressed to the followers collection of the author, at minimum. When NodeBB sees this, it doesn't even consider it public, because the public pseudo user isn't addressed. We also have no concept of "followers only", but we handle them just fine.