I too have been using Unix long before MacOS. A pure Unix kernel is not a hybrid product like Mach (for older implementations) and XNU. Similar does not equate to being identical. For example access control and sandboxing is quite different.
Different than what? It sounds like perhaps you just don’t have much practical experience with Unix beyond Linux. Nobody with Unix background would ever use the phrase “pure Unix" or think that I was talking about systems being “identical” (a word I never used or even implied).
I remember back in the mid-80s a co-worker of mine had a little sticker on his monitor that read “Remember: Unix on some systems is nUxi”. We were doing a lot of integration work based on Altos Xenix back then (for MAS-90 accounting systems) and it was one of the weirdest Unixes I’ve ever had to work with.
macOS
is UNIX®. Linux is not (heck, it’s even in the acronym GNU, right?). But I’m happy to admit that UNIX® certification is more of a marketing gesture than a practical difference.
On the scale of weirdness, though, macOS doesn’t even come close to the top of the list when held up against other Unix outliers like AIX or HPUX. Every Unix vendor lays their own bizarre ideas on top of the core POSIX experience, but by and large the “Unix” experience on all these various systems is unremarkably familiar. (I mean, once you learn that killall in Solaris does exactly what it says it will do).
By and large, macOS is a fairly vanilla Unix experience. Tying back to the thread, it’s why I dread leaving the platform. macOS is (in my view) the best Unix desktop experience you can get these days. That’s what I need to do my job and what I prefer for my personal use. Unix on my desktop with the fewest compromises. Linux has made great strides in the past few years, more rapidly than I can ever remember. It feels like the dynamic may be shifting finally (how long has Slashdot been promising us the year of the Linux desktop? They may have been a decade off, but I’m allowing myself to be optimistic now at least.
If I ever do finally get driven off the platform by Apple’s bad decisions it’ll be Linux where I land. But at least today that choice comes with a lot of sacrifices I’d prefer to avoid.
Microsoft’s Linux subsystem for Windows is a noble gesture, but it’s a long way from what it needs to be for my use case.
Edit to add: Here's a shot across the bow from Microsoft. Apple needs to get its act together, and fast.