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You can even use it under Linux if you wish btw, you just can't bind to it, but you can have your firewall redirect port 0 to something else.

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You can bind to it on some versions of Linux. I've scanned a bunch of Linux systems that host stuff on port 0.

Your observation doesn't contradict the use of firewall rules to accomplish this.

It's not some ufw rule that normally prevents hosting a service on port 0.

That's not what was said. They said that a firewall rule can redirect traffic coming in on port 0 to a running service even when a service cannot bind directly to port 0.

Binding with port 0 as argument for AF_INET binds a random available port, not port 0. This is documented behavior of Linux and likely every other OS implementing a BSD-style socket interface.

Also note that ufw is just a tiny, non-standard wrapper for the much more powerful nftables/iptables interfaces

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