that's based on a mistaken assumption.

ActivityPub servers don't really pass traffic to each other unless they have a reason to, and that reason is usually followers.

unless an account on mostr.pub is following an account on another AP server, mostr.pub isn't going to receive traffic from that account, so there is nothing to forward to Nostr.

and unless an account on another AP server follows an account on mostr.pub, that AP server won't see any traffic from mostr.pub about that account.

mostr.pub basically passes traffic between follows and followers on either side, and nothing else. everything that comes from relay.mostr.pub is on the Nostr side there for a reason.