I would guess that AMD just opened up all specs of their GPU to Sony and Sony did most of the driver/API design along with some help from AMD. You also have to

I would guess that AMD just opened up all specs of their GPU to Sony and Sony did most of the driver/API design along with some help from AMD. You also have to consider that the PS4 graphics APIs will likely be a lot simpler and down to the metal than DX9/10/11 and (especially) OpenCL.