You can also find commits from Apple employees in Clang, CUPS, LLVM, Xorg, WebKit, etcetera. These might not be things that you appreciate, but they do exist and other people do appreciate them. I notice that Dave Airlie is on the forums. You could always ask him about Apple's contributions to Xorg. Of course, that assumes that you actually use Xorg. If you do not, I could understand why you would not care.
With that said, I consider the concept of a derived work to be questionable in the context of the Google versus Oracle lawsuit and concepts such as ReactOS. If it is fine to be compatible with interfaces, at what point can you call something a derived work if you are able to replace both sides of an interface? Granted, you do not always have the other side of the interface implemented, but the logical necessity of this seems similar to the necessity of displaced water when demonstrating the buoyant force. FreeBSD's kernel does implement some Linux kernel interfaces. Had the company in question foreseen this situation, they could have done their work on FreeBSD and produced either the same result or a similar result for Linux.