My guess is that it is not testing
Lots of the "efforts" (though they have been doing this on consoles for ages, possibly in a less controlled environment) will be on the devs. They stated that they've been talking about it for a while already. We saw a pretty advanced demo.
I think it will be tied to a certain OS. They could be communicating that early because they want plenty of games and software to make use of this at launch (vs the usual slow adoption of new API).
If they release an OS that replaces Windows phone and RT, that runs both on ARM and X86 they may want a lot of things (/software and tools) to be ready not only gaming related to maximize the final announcement.
I would think that direct x12 games are going to be "apps" using
Windows runtine 2.0.
Virtualization on the Xb1 makes sense in that context.
So it could be that the extra time is also to iron things out on the CPU side.
EDIT
to some extend the XB1 looks like a gigantic beta test for such a move. Having the hypervisor doing its things, the OS handling background tasks on CPU and GPU compute (kinect) and the sand boxed games also using the GPU.
Next step is having a programming model for the cpu that is as ISA agnostic as possible and performs as close as possible to "native" programs.
EDIT 2
I also wonder about the desktop of Windows, it is otish but I think it could be a goner on some version on their next OS. It could be open in a virtual machine /sandbox for legacy support and pro /specific usage.
In a world in which more and more people are fine with ChromesOS, I think the desktop is no longer mandatory for windows.