How would you explain roundabouts having consistently better throughput, even when compared to traffic lights?

Security through obscurity is a very valid way to secure something. It shouldn’t be the only way of securing something, but it can be a valid additional layer.

The knock sequence is a secret, just like a password. It may not be a particularly strong secret, but is is strong enough to keep out casual attackers. You’ll still need additional security, but sshd is well equipped to provide that.