Nice to see WireGuard support moving forward, and an update to recent OpenSSH. They were running an OpenSSH that was 3 years old (7.9 was released in October 2018).
Just in base, you could switch to openssl from ports in a minute.
openssh and openssl don't have shared code and are incompatible protocols on the wire, even when they use the same cryptography (e.g. X25519-Ed25519-ChaCha20-Poly1305).
You can install the newest openssh-portable from ports too.