I think "attacker has a private key" is an unreasonable threat model to protect against, not least because the key is so much harder to crack than port knocking. The benefit to port knocking is against information disclosure ("this server is running Apache httpd version x.y and sshd version z"), brute force, noisy logs, and pre-auth vulnerabilities (things like heartbleed and shellshock). While in theory I suppose wireguard could be affected by pre-auth vulnerabilities and maybe a completely blind bruteforce attack, it's a listening UDP port that doesn't respond until it sees valid credentials, so it's completely invisible to an attacker.