Maybe I missed it in reading the article but what, specifically, is not available to developers if they choose to write the application against the typical Win32 APIs?

Are they, for instance, making certain hardware accelerated APIs only available for UWP apps? If so, are these APIs designed in such a way as to prevent someone from creating an API that runs on top of Win32 that exposes the capabilities (i.e. by requiring drivers be locked down in such a way as to disallow interaction outside of UWP)?

I can't see a way outside of driver feature lock-out that the OS could effectively prevent game developers from developing games the way they have been in the past. I'm guessing this has a lot to do with their "Write it for Xbox One and it'll work on Windows 10, too" goals, but that feels more like a convenience than a lock-in -- games are a constant exercise of porting to get them on each platform, so getting them onto a non-UWP target would be another platform (which many developers would simply skip since UWP would be theoretically easier).