WireGuard default UDP port is open on my home router. This website says 404. Why? Because the scans don't scan full UDP port range, is my guess. I trust this with regards to TCP.
“One design goal of WireGuard is to avoid storing any state prior to authentication and to not send any responses to unauthenticated packets. With no state stored for unauthenticated packets, and with no response generated, WireGuard is invisible to illegitimate peers and network scanners.” https://www.wireguard.com/papers/wireguard.pdf
Yeah, It does not scan all ports, just a select thousand or so, and 51820 does not seem to be among them
More importantly WireGuard doesn’t reply to unauthenticated traffic so it won’t show up on a scan.