> and then various other Mac layers (GUI, Cocoa, etc).

Incorrect. The GUI wasn't ported, it was rewritten for OS X. Cocoa's heritage is NeXT to the core. Nothing vaguely resembling Cocoa existed in the classic Mac environment.

  > a pulled-in, non-NeXT BSD application layer

Incorrect. There is little of "BSD" in the Mac OS X environment, save for some FreeBSD userland components, most of which only matter on the command line. The application layer of Mac OS X is Cocoa.

(There is also the Carbon application layer which does partially derive from classic Mac, but that existed solely to ease application porting of existing Mac apps. But even Carbon is a hybrid, and was actually back-ported to Mac OS 9.)

  > Technically there is nothing wrong with his statement.

Any Mac OS X software developer will know enough to confidently refute the accuracy of Gates' claims.