[skissane]

IBM z/OS is officially a Unix-a very weird Unix which uses EBCDIC-but it passed the test suite (an old but still valid version, which makes it somewhat outdated) and IBM paid the fee to The Open Group, so officially it is a Unix. (Although somewhat outdated, they recently added a partial emulation of the Linux namespace syscalls-clone/unshare/etc-in order to port K8S to z/OS; but that’s not part of the Unix standard.)

If Microsoft had wanted, Windows could have officially been Unix too-they could have licensed the test suite, run it under their POSIX/SFU/SUA subsystem, fixed the failures, paid the fee-and then Windows would be a Unix. They never did-not (as far as I’m aware) for any technical reason, simply because as a matter of business strategy, they decided not to invest in this.