These are all layers on top of the lower-level OS, which is the only part that would need to be ported; and it was already ported.

Quartz is basically "Display PDF" instead of "Display PostScript" with the architectural difference being that DPS was client-server, with bits of PostScript being interpreted on the fly; while Quartz got rid of the round trips between client and server, merging everything into one faster, monolithic layer. I recommend you look at the PS and then the PDF specs, they are about 90% identical.

The code imported from MacOS also ran in a separate "box" on top of the lower-level details.

Really the best case I can make for my argument is to point out the similarity between NeXTStep's IOKit drivers and the OSX IOKit drivers. That is the part, that really DOES interface with the guts of the system.