So its about technical guidance, not approval to include code. Got it.

WireGuard is a registered trademark.

This puts reasonable restrictions on the usage of the term WireGuard to name your alternative implementation, but neither prevents other people (without any approval) from implementing the protocol nor even from using the term WireGuard to (correctly) describe or label the protocol implemented (and I even could see a solid argument that any command line tooling that might be expected to be compatible with existing third-party tooling that calls out to said tools should be allowed to use the term "wg" or "wireguard", given prior legal precedent that if you use a trademark in a place where it causes a compatibility issue you cannot attempt to claim infringement of that trademark to later stifle interoperability with your project; but, if you go down this path, I'd highly recommend consulting an actual lawyer and getting your response ready to any complaint).