Yes, but that port needs to remain "open" for the legitimate sshd traffic. Can you see a difference in ownership as the firewall sees it between sshd and some user daemon? Sshd drops root partially when login succeeds.
that has nothing to do with sshd's listening socket, which remains owned by root
Sure the listening one remains owned by root. But the connected one? If you limit packets from/to e.g. 2222 to uid 0, will legitimate ssh traffic work? I don't say it won't, genuinely unsure. Haven't tried and today is a holiday. Maybe tomorrow :)