AFIK, the idea is that the (main/audio) menu will be "the same" in general appearance as the "popup" one shown - when you put in the disc, you'll get the menu + background clip. When you move to "audio," for example, you'd see the bar with the options there. After picking a track, you would then start playback. The "popup" is simply for changing during playback.
For example, say you wanted to hear commentary - you'd start it with that. Say you got bored of it. You could easily switch back to your language / track of choice during playback.

My initial idea for the menus was more "full screen," but I do have to say that the idea of a simple bar that covers less screen area is arguably a much, much better choice for playback, and will also suit the "main menu" screen nicely.