I'd be more interested in understanding why they chose to implement their own file system instead of adopting something like ZFS.

No doubt there is reasoning in there besides the "we are an evil company" logic - Apple developers are people too...

I just do not understand rationale. File systems is hard stuff, instead taking something that is probably best file system around they decided for long and painful road. Note that they worked before on OpenZFS in Apple ....

ummmm... metal cames out 2 years before vulkan. Vulkan was inspired by metal.

nope, both are inspired by AMD Mantle.

Apple not only ignored first major consensus in Games industry it also blocked support on its devices. It also stuck with old OpenGL. This will make gaming on Apple to suck for many years to come.