I’d like to try it but why force users to login with either gmail or Microsoft? What’s wrong with a plain old username/any other email provider and password?

They addressed this in another post on HN.

Basically they don't want to manage U/P. They are looking at other services as well (such as github).

Yes, its omission from the article seemed like an important oversight to me.

I see it in the "Other WireGuard goodies" section.

I stand corrected, or it was added after I looked at the page (which appears to have been updated on April 8).