They already had a low level graphics API for ps4, GNM, before Mantle. In fact, EA DICE's Johan Anderson who did most of the groundwork for mantle, said he got his ideas from working with console APIs. At the time he was working on the mantle spec it probably would have been the xbox 360 or ps3 APIs or an early form of the ps4 API. The X1 didn't launch with a low level API, but they provided one some time before 2014 according to a Metro Redux dev