Indeed they did, but this is a device still in development, to be released late 2020. I would not be surprised if the Windows 10 X team is working to make it at least possible to run 10 X on the smaller device. I'd much prefer 10 X with an Android subsystem. But like I said, I'll pick one up regardless.
I'm sure it could run 10 X easily enough, the problem is the same problem that killed the Windows Phone ecosystem. No one's developing mobile/touch apps for anything other than Android and iOS. Windows has been fully touch enabled for years now but still there are almost no apps that take advantage of that, there isn't even a decent kindle app in the microsoft store. I wish this wasn't true but the truth is mobile belongs to Apple and Google and tablets belong to Apple.
Which is why I'd like an Android subsystem. I want the basic UI experience and default apps to be Windows 10 X. For 90% of my phone usage, I'm using the stock apps (launcher, email, browser, calendar, etc.), and I'd prefer that to be the 10 X experience rather than Android. I want Android to feel like an app safety net, not the primary experience.
But like I said before, even without that, it's still the right device for me.
Yeah, but that raises the question as to what's the upside for Microsoft for building the Duo? Why divert expensive engineering resources to become just another niche Android phone vendor and help shovel money into Google's pockets on top of that?
The only thing they said is that they "are building upon Android".
I think it could both means that they are using Android as an OS or that they built some kind of WSL-like subsystem. The latter scenario is more interesting because it would open up the possibility to run UWP apps that can also run on Windows 10 X, both leveraging the two screens.