The Game Porting Toolkit contains a proprietary (143 MiB) D3DMetal framework that directly translates the Direct3D calls to Metal. They don't use the Direct3D --Wine--> Vulkan --MoltenVK--> Metal path that opensource Wine/CrossOver Wine uses.
"you are granted a limited, non-exclusive, non-transferable, personal copyright license to (i) install, internally use, and test the Apple Software for the sole purpose of developing, testing, or evaluating video games for use on Apple-branded products"
That does look like they have their own shader ir converter. But the fact that there's d3d9-d3d12 there, and under a wine directory, means those parts, at least, are borrowed.
That this exists is hilariously self-defeating. Linux gaming is becoming Proton-driven, so with this and VKD3D, we're potentially looking at Direct3D 12 as the most widely supported cross-platform API. Why write Vulkan or Metal when D3D runs everywhere now?