The time table is being pretty generous to Apple. Metal, Vulkan, and DX12 are reworked versions of Mantle.
The entire point of Mantle was to be a proof of concept that could be reworked into cross platform API (which became Vulkan), there was plenty of work already being done by Khronos in 2014 (and Apple knew this). And they just went out and released Metal anyway.
I also blame Microsoft for the same thing, early parts of the DX12 docs were taken word for word out of the Mantle docs, that's how similar they are. But Microsoft at least had couple decades of having a competing API, but Apple went out to create a new one for some reason.
Talk about rewriting history, Mantle wasn't never supposed to become Vulkan, it only happened because AMD was generous and Khronos would otherwise still be thinking how OpenGL Next would look like.
The entire point of Mantle was to be a proof of concept that could be reworked into cross platform API (which became Vulkan), there was plenty of work already being done by Khronos in 2014 (and Apple knew this). And they just went out and released Metal anyway.
I also blame Microsoft for the same thing, early parts of the DX12 docs were taken word for word out of the Mantle docs, that's how similar they are. But Microsoft at least had couple decades of having a competing API, but Apple went out to create a new one for some reason.