The non-standard port is of trivial value, but it's practically zero cost- that's why it's used. Port knocking doesn't have that benefit- you're establishing another secret that has to be maintained and accessed- but unlike key based auth or passwords- that secret is insecure in transit and unwieldy to use.

If you want to add another layer and manage another secret- why not just add another layer of the lower-friction and more secure methods we already use to establish secret-based auth?