They still have some pretty old version of OpenGL and Metal was a bit before Vulkan, so it’s sort of a lightning vs USB C situation.

I don’t believe that it was easy. Since it started macOS was based on BSD, not Linux, which is quite different. They also use different types of binaries and the similarities between kernels should end beyond the BSD compatibility layer. See https://wiki.freebsd.org/Myths#FreeBSD_is_Just_macOS_Without_the_Good_Bits