So is z/OS. UNIX branding under The Open Group is correctly focused on the user's perception and has little to do with kernel implementation. Mach is is no more UNIX than NT is VMS, to call one the other in this context of kernel discussion is reductionist and impedes correct understanding of the historical roots and critical differences.
OSF/1 however was a complete Unix system, even if it based its kernel on Mach (at least partially because it offered a fast path to SMP and threading), and formed the BSD side of Unix wars.
And NeXTSTEP didn't diverge too much that when OSX was released they updated the code base with last OSFMK release.
I'd say SunOS (4.x and earlier, not Solaris) was the BSD side of the Unix wars. For most of the 90's, SunOS was the gold standard for a Unix workstation. I worked at a couple of early Internet providers and the users demanded Sun systems for shell accounts. Anything else was "too weird" and would often have trouble compiling open source software.
Correct, SunOS is a descendant of BSD and was the most widely used and renowned one during that time. And like you said, it was the gold standard for easy builds of most contemporary software and enthusiastic support.
DEC Ultrix is also BSD kin and IBM AOS and HP-BSD were intentionally vanilla BSDs. There were some commercial BSDs like Mt Xinu and BSDi that were episodically relevant.
BSD proper was alive and well especially in the academic and research circles into the 1990s and we get the current derivatives like Net, Free, and Open which are direct kin.
Mach is regularly BSD-affined because BSD was a typically ported server but Mach is decidedly its own thing (as a simple and drastic counterexample, there was MkLinux and OS/2 for PowerPC which had little to do with BSD but are very much Mach). NeXTSTEP and eventually Darwin/macOS inherit BSD affinity.
It had some heavy hitters behind it, but my understanding is DEC's effort was mired in organizational problems leading to fractured strategy and commitment. Many vendors were still figuring shared libraries out into the early 1990s so it must have been swept away from Ultrix once OSF/1 became the plan of record.
Again, so too is z/OS a complete UNIX system (from the user perspective).
OSF/1 is decidedly not the the BSD side of the Unix wars, it is its own alternative strand against its contemporaries BSD and System V. More specifically, it took its initial UNIX personality from IBM AIX and was rapidly developed and redefined to accommodate the standards du jour which included BSD and System V APIs.
