There are modified Darwin kernels that allow Hackintosh to work on AMD processors. These kernels have some stability issues, but if hobbyist outsiders can get most of the way, I don't forsee it being a big hurdle for actual Apple engineers.
As I see it, as long as Apple is putting out x86 hardware, there’s no reason why it can’t be AMD x86 hardware.
(I’m also secretly hoping the ARM thing won’t actually happen, but that’s neither here nor there, and I’m probably wrong.)
I also have major concerns about raw performance at the high end, and I suspect ARM would come with even more software lockdown, although there's no reason that has to be the case.
I subscribe to the theory that the Air will move to ARM at some point. Adding this feature to XCode sounds like the sort of thing you would do to prepare the way for an architecture shift. Especially if you were still on the fence about that shift. Let's just get a feel of how viable this space is before committing to anything.