They also published some architecture descriptions/images. I find this most interesting:
As you can see, compared to the initial illustrations, there is now a middle layer where "platform" vendors can plug in their own runtime system. From the application side, there is a uniform standardized interface applications talk to, and from the hardware/device side there is a unified device layer that drivers talk to:
It seems like a clean architecture from a software perspective, and like a very promising design from a consumer perspective. It should allow you to mix and match applications, runtime systems and hardware devices to get the VR experience closest to what you want.