There are android phones that can output a different screen than that of the phone. My moto atrix from the top of my head, so may not be impossible.
But from your answer another question arises, can canonical extend android's DS enough that it would also work well in the desktop form factor?
My point is: Canonical is faced with a choice: They can follow intel and the other GNU distros and adopt wayland, relying on Intel and community to further develop it, also requiring porting of the desktop GPU blobs to this DS supported only by GNU distros and also of the apps/toolkits. Or, they can follow google and adopt android/chromeOS DS and instantly gain support for ARM SoCs drivers, which would require convincing AMD and nvidia to port their blobs to android/chrome OS, the porting of the apps and toolkits and rely on google to further develop the DS.
Neither is painless, but I can understand canonical if they choose to stand closer to a google/linux instead of GNU/linux. Google has already taken linux where no other GNU/linux distro has ever gone: consumer devices. By "borrowing" hardware support from android/chromeOS, ubuntu can deliver a better user experience and has a higher chance of convincing an OEM to ship a product with it.
Maybe wayland is technically better, but that is meaningless if it only performs well in a very strict number of devices.
Why don't port their desktop to android instead. As I understand it, this is was firefox did with firefox os.