Apple's attitude towards other OSes running on their hardware is less "supportive" and more "barely tolerates". Also as a general rule Apple doesn't contribute much to open source outside of some high profile projects like Swift and Webkit.

As somebody using a Linux distro compiled with Clang, I consider the work on LLVM to be Apple’s greatest contribution to Open Source.

I have seen others say that CUPS is.

And there is libdispatch, mdns, etc.

Anyway, they contribute more than you think.