macOS (neé OS X) is actually more Unix than Linux, as it is certified.
That certainly says more about the certification process than about unix-ness of macOS ;)
On the other hand large fraction of UNIX(TM) certified OSes are in fact niche Linux distributions (IIRC mostly RHEL derivates).
In all it is mostly about nobody giving a damn about the certification outside of very specialized applications.
At first I thought he meant server-side, but OS/X is as Unix as Linux is or even Solaris was.