I'm not sure what they are using on Android, but crostini on ChromeOS is an LXC container. They call the LXC container a VM in their help and doco.
As for "different Windowing systems", what they describe:
Instead, Google envisions Linux apps living side-by-side with native Android apps in Android’s desktop mode.
is what corstini does now. It mostly works. For example, it works well enough that Mozilla's recommends you install Firefox on ChromeOS using .deb's into the crostini VM. But there are lots of UI paper cuts - for example model dialogue boxes often come up a few pixels high and wide. Because they are modal the app appears to freeze, and because they are tiny you don't always see them so you don't know why the app has frozen.
If they fixed all that, all we have left is the horsepower issue. Debian programs need at least 8GB, preferably 16GB. Current ARM offerings feel distinctly under powered, but I suspect Debian running on Snapdragon-X under Android would be very usable.
