The author says this, but oh boy you should see how many individuals on /r/macos hinge on "It's good cus UNIX!", not that they understand what a UNIX is or where to find one.
The certification isn't useful. Apple doesn't sell to the types of organizations/governments that would care. And macOS is so limiting in what you can do with it in comparison to not-UNIXes, branding it UNIX is a waste of time.