It sounds like it was just a bad example. Feels like they just wanted to show how to do as much as possible even though it didn't make any sense for the app rather than it being required of all apps. If it were required for all apps I feel like they would just do something sneaky and make you not have to explicitly set the CSS width to 120%; they could just do it at build time or something.
Let me put it simple: the example app is forced to scroll horizontally by explicitly setting the total CSS width to 120%. There is not a shortage of space here, mind you, so scrolling really isn't justified. In fact, let me quote the PDF:This is in the first chapter. I could understand if the app tries to show a lot of stuff at the same time (then you may want to add scrolling to give more room for stuff), but this isn't the case of this app. Maybe that's the point it's trying to convey, but the way it's explained makes it look like all apps are expected to implement scrolling regardless of whether it's really needed or not.
It is common in Metro apps (or at least common in the ones developed so far) to provide a content area that is wider than the screen and allow the user to scroll from left to right to access different regions of the app. Setting the cumulative width to 120 percent sets up that behavior, which you will be able to see when I run the example web app later in this chapter.
No idea though. There is definitely something off about the example in general, but I really wouldn't see them putting the responsibility on developers to do something so trivial but easy to forget in order to enforce a standard.