> However, there are a number of operating systems, such as MacOS, that are fully compliant with the POSIX standard and can therefore be called Unix operating systems, not just Unix-like.
It's a bit more nuanced than this. Being fully compliant with the POSIX standard isn't quite enough; you have to pass an official conformance test suite and be certified by the Open Group to have passed it, in order to call yourself "Unix", rather than just being Unix-like.
So there could be (and likely are) implementations out there which are fully compliant with the standard, but have not chosen to get certified, and are therefore technically still "not Unix".
It's a bit more nuanced than this. Being fully compliant with the POSIX standard isn't quite enough; you have to pass an official conformance test suite and be certified by the Open Group to have passed it, in order to call yourself "Unix", rather than just being Unix-like.
https://en.wikipedia.org/wiki/Single_UNIX_Specification
So there could be (and likely are) implementations out there which are fully compliant with the standard, but have not chosen to get certified, and are therefore technically still "not Unix".