It seems SemiAccurate was...accurate - Microsoft just took Mantle and renamed it DX12, to call it their own "innovation":
http://semiaccurate.com/2014/03/18/microsoft-adopts-mantle-c...
http://semiaccurate.com/2014/03/18/microsoft-adopts-mantle-c...
- reduce raw CPU work by increasing precompilation of state and data to native formats
- reduce communication from the CPU to the GPU, by increasing the flexibility of API calls
- reduce CPU <-> GPU dependencies by moving some forms of logic and flow into the GPU
All of them conspire to increase parallelism as well since there are in general fewer points where synchronization is needed.
Playstation developers have a long tradition of doing these things since the first model; Xbox developers not so much, but still a lot more than on PCs. Since the general goal is to reduce API surface, remove abstractions and match the metal more closely, it's natural for APIs to converge.
It's something of a Direct3D tradition to design the API around one hardware vendor's state of the art. Direct3D 9 was pretty much based on what the ATI Radeon 9700 could do. At the time this was a serious inconvenience to NVIDIA who had taken a different tack with the Geforce FX (I think it was -- it's been a while).
Out of all such rumor sites (Fudzilla and BSN being two other large ones), SA is the only one worth taking seriously.