I didnt say that either. Do you seriously think that the xbox one api bears no relation to the xbox 360 api? And who codeveloped that?
At its core, DX11.X, DX12 and mantle all share a common root... Directx, which has always been a collaboration between all three.
Did AMD sit down with a completely blank piece of paper when they started work on Mantle?
Of course they didnt, they borrowed heavily from over a decade of previous work from other people.
Obviously, working on xbox one with GCN hardware made it easier for them to take that work and apply it to other GCN hardware, hence why they were able to rush out mantle while at the same time MS have had to bear in mind compatability, and by the sounds of things add other features for next gen PC hardware.
But no, I'm sure you are right and MS only started work on a much more complex project a long time AFTER Mantle was with developers and still manage to get it in to developers hands around a year later. or in fact within a couple of months to be able to do the forza demo.
Thanks for the laughs.