I'm sure that others here are well familiar with other officially certified UNIX operating systems[1], but given many widely used Unix-like systems not on the list, I don't think Apple gains anything from the certification. Moreover, it probably is deleterious, forcing them to adhere to system designs that made sense in the 70s and 80s.
I'd be surprised if Apple spends all that much money/effort on official Unix certification, precisely because I doubt Apple would bother. How many customers are choosing macOS over something else because of the Unix certification? In a world where Linux exists?
(This discussion, of course, has nothing to do with whether or not macOS adheres to Unix principles or compatibility.)
1: https://www.opengroup.org/openbrand/register/