Why it was rejected? Because Microsoft was using Mantle as the base of DX12. What is Mantle is essence? What is DX12 in essence?
Mantle is this: Low CPU overhead, Asynchronous Compute, HSA 2.0 capabilities, explicit multithreading, and direct control over hardware, graphics and compute united into single queue.
What is DirectX 12? All of above plus Direct3D 12.
What is Vulkan? All of above plus OpenGL.
What is Metal? All of above, plus bits and pieces from other APIs like OpenCL, D3D10, D3D11.
Lets stop this, its Zen thread.
It was rejected because Microsoft were already working on their own API. Having the same feature set doesn't equate that both API are the same or that one as inspired the other.
You want to stop this, easy, stop talking crap and reposting old already debunk junk. And in any case why would there be an AMD Zen in this forum section when no Mac Pro are using them and there isn't even a glimmer of hope that any will? Yea, it's just another of your AMD circle jerk who would be more at home on /r/AyMD on Reddit. Try creating thread relevant in this sub forum for a change instead of a vehicule to post AMD sponsered propaganda and anonymous blog/forum post about AMD.