> Who cares whether a Unix is certified? Linux is the big daddy of the server rhythm these days
Linux has a huge installed base, absolutely. Most of my work throughout my career has been on Linux. We also know that GNU stands for Gnu's Not Unix. Linux is popular, and it's explicitly Not Unix. There is no guarantee your Unix software or integrations will continue to work on any particular version of any particular Linux distribution, as they try out a third init system in as many years.
So who cares about certified Unix? Two groups of people. People who have enterprise production systems running Unix software that MATTERS care. If you're running a payroll system for 10,000 employees and a glitch means missing a pay day, or perhaps ending up with the decimal point in the wrong place on everyone's pay check, certification of the whole stack is good. You can, at a cost, show that the software uses only official Unix apis, and will therefore run on any certified Unix. Similarly , regulators and such like certified components for similar reasons.
The second group is represented by alot of the systemd comments. Certified Unix means you have certain guarantees about how things (still) behave. You won't have important stuff changed out from under you, if you interface with the system as a Unix system, not as a Brand X version y.z system. Apple CAN'T fuck certain things up in the next version, systemd style, without losing their certification. That can be attractive to a lot of people.