In the same presentation as you linked, on slide 9, you can see Apple was a part of the Vulkan effort. The subsequently withdrew from Vulkan, before releasing Metal.
WWDC 2014 in June had the 'Zen Garden' demo running Unreal 4 on the Metal API [1], and it was released to the app store in September [2]. In a different session, CryEngine was shown running Metal on an iPad [3]. That's two high-profile game engines already using the API. A limited release perhaps, but I count that as a significant head-start, all the while Khronos was just organizing glNext.
