Yes. Well, since 2002 some old versions aren't, but v7 onwards is. macOS is based on XNU and NeXTSTEP, which are based on Mach and BSD, but macOS is not solely a Unix system and few use it as such. In fact, if you were to make a program using only the FLOSS unixy parts of macOS, porting it to other POSIX systems would likely be trivial. I hardly even count that as "macOS software" in the exclusivity sense.
macOS being unix-based doesn't make it less proprietary. z/OS is Unix certified and proprietary as balls.
People forget that actual Unix descendants are really mostly proprietary. Linux and BSD are what we often think of today, but those are really more "Unix-like's" and many of the "true" Unixes tend to be proprietary systems like AIX, HP-UX, IRIX (now deceased), z/OS etc.
The bare core of macOS (Darwin) is open source, but pretty much everything people identify with macOS is proprietary code built on top of Darwin. You used to be able to get standalone distributions of Darwin (maybe you still can?) that people built from Apple's source and they were almost completely unrecognizable.
Right, that's my point. What most people associate with "macOS" is actually the stuff layered on top of Darwin, which is mostly proprietary. It's not just the GUI, it's also a lot of the frameworks and libraries as well.
macOS being unix-based doesn't make it less proprietary. z/OS is Unix certified and proprietary as balls.