Openssh allows: password auth with weak passwords, PAM with potentially insecure PAM modules, gssappi, configurable ciphers, etc. Wireguard has none of those.

Each of those auth methods adds more possible vulnerable code paths and accidentally insecure configurations. It's very reasonable to believe that wireguard auth, since it has far fewer moving parts and configuration knobs, is likely to have fewer ways it can be insecure.