I think he/she meant - the exact kernel doesnâ(TM)t really define if an OS is âoeUNIXâ or not. There is truth and fallicy to that. A POSIX kernel presents a single file tree, and plain, directory,, block and character files. Those are pretty germaine to itâ(TM)s definition. It also provides file descriptors, (pipe primitives) and a well-defined set of basic syscall interfaces. It would be hard to argue a kernel supplying all this wasnâ(TM)t âoeLinuxâ.
That said - there are a ton of user-space programs which are accustomed to âoeUNIXâ systems. And they all obey a central philosophy of having many small programs which can/do communicate by standard file descriptors, etc.
But in any respect - to the original point - MacOS obeys every one of the things I mention verbatim.