UNIX is a spec. The point of being a "certified UNIX" is that the system conforms to SUS/POSIX and software developed targeting those specifications and their API are expected to run.
One might question the usefulness of SUS/POSIX in a world where most of the *nix ecosystem targets GNU, but saying it has no purpose beyond advertising is disingenuous.
One might question the usefulness of SUS/POSIX in a world where most of the *nix ecosystem targets GNU, but saying it has no purpose beyond advertising is disingenuous.