Writing a backup tool and supporting an OS properly is a lot more work than just writing portable software that can run on the portable standard APIs supported by an OS. People expect their backups to preserve file attributes, access control policies, etc.
Beyond traditional POSIX file attributes like ownership and access mode, there are POSIX ACLs. Linux also has extended attributes and SELinux policies. Windows has a different ACL model, and NFSv4 ACLs on Linux are more like those. A Mac has "resource forks" which have no real analogue in Linux or POSIX systems, and a backup/restore that ignores these would be very useless to most Mac users.
While I agree, that macOS is a UNIX, backup tends to be a very sensitive form of software (think scalpell, not hammer) and I would not dare to use it on a platform without certification by its vendor (in this case an "OK" by the author).
We do not live in a perfect world. But backups can have exactly zero errors.
But thanks for pointing out, that a mac brew exists. I did not see this.
If it is targeting UNIX then it must work on Mac. Echoing your words, UNIX has stringent requirements to be UNIX and macOS fulfilled those requirements.
MacOS IS UNIX.
As a side note for fun, Linux is not UNIX and that’s intentional. Freedom is nice. ;)
Linux is not UNIX™, but it is a Unix in most meaningful senses of that term. My opinion would be that making much of the distinction would be pedantic in a negative sense; there does not seem to be a practical reason for this.
Also, I can't imagine how something could be for Unix-likes and not target Mac OS, since it is literally a certified Unix.