I'm pretty sure that the PowerPC version of NT never ran on a Mac, just some IBM and Motorola systems.
More to the point, it was discontinued after NT 4.0 (July 1996), which was released seven years prior to the start of the Xbox 360 project (Feb 2003). Also, according to Wicked-Pedo
, Microsoft denies that the XBox and Xbox 360 kernels were based on any version of Windows at all, though a subset of the Windows APIs were implemented for it.
This doesn't mean that the prototypes weren't using a PowerPC version of the NT kernel, or that they didn't have a purpose-built version of such a kernel running on a PowerMac, but it does seem unlikely. If anything, they were probably running it in an emulator until a prototype developer SBC could be built. I don't know offhand how much the Xenon differed from the base Power architecture
of the early 1990s, or from other implementations of it such as the PowerPC G5 or the Cell PPE used in the PS3.