I would think Windows could run on a Cortex-A7, no? Perhaps the specific parts they're using have a ridiculously-low clock rate or other painful limitations?
Cortex-Ms typically can't run linux (excepting uClinux) as they don't have the RAM and typically don't need to as they address a different need (dedicated function instead of general-purpose compute), and have far fewer peripheral interfaces. It used to be all about power profile, but the recent SoCs are getting pretty competitive there as well.
But as I said the traditional segmentation at this end of the spectrum is being re-cast seemingly every second week, and so terms such as 'microcontroller' are becoming less meaningful all the time. And who the hell can agree on just what 'embedded' means these days?
Or what it'll mean in 10 years, as power/radio/processor/sensor specs continue to improve. I'm speculating, but perhaps MS is banking on the low-end to grow, up into the Android/iOS space. So instead of our current 3-10 devices per family we'll have 30-100 devices. Hopefully, those devices will be secure. Maybe they'll be useful :-)
FWIW, my personal definition is "doesn't have an MMU"