The video shows a "Hello World" app being created simultaneously for iPhone, Windows Phone, and Android. But the point here is not about phone platforms, that's just a way to create an equal playing field.
The code to implement the "logic" of the app is essentially the same on all platforms. However, in Visual Studio it only takes about 35 seconds to accomplish the task at hand, since there is no additional "plumbing" code needed to make it work.
Obviously this is a trivial example, but imagine how the trend would continue on a more complex app. Would you rather spend your time improving an app, or just making it run?