Classics Mac OS isn’t based on NeXTSTEP; Mac OS X is, and the Mach kernel in particular is much older than Linux. It takes like thirty seconds of googling to get your history straight.

Me neither. In 1997, Apple bought NeXT to get Jobs back. In the bargain they acquired NeXTSTEP, which was, by then, a mature and robust OS. Once Jobs was back in charge at Apple, he deemed classic MacOS unsalvageable, and decided to move the Mac to NeXTSTEP - an OS they already owned and which he was obviously familiar with. It took a few years to build what would be known as Cocoa and Carbon, and eventually OSX. Linux was never even in the picture.