mostr.pub only knows about the posts that have been sent to it.

basically some nostr user has to follow a fedi account in order for it to let the server that fedi user is on that it would like to receive those.

It's similar in reverse, a fedi user has to ask for the posts of a nostr user from the bridge in order for it to send them along. It won't do that unless it thinks there is a follower relationship established