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:

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.

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.
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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.

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.

Yes, the book tries to do a "here's how you do everything" example app. The problem is that obviously the message got lost there because the way it's explained looks like every app has to do that (the author didn't provide an explanation what's the idea behind it), and it's likely that many developers who learn from it will indeed think that's the right thing to do regardless of the situation.

Also Metro apps (at least the kind explained in this book, dunno about the non-HTML ones) are running on the IE10 engine, so technically you have to provide all the CSS. There's a default CSS added to the project for you to use, but you can do pretty much anything you could do in a website, since the same rules apply (the only exception is the extra stuff that's Metro-specific, but that doesn't conflict with anything else).