I did a quick check with Windows Media Centre, Plex. MPC-HD, and VLC and not of one of them responded to the double-click for changing the UI. Alt-Enter is the clear standard for that. Maybe you meant during video playback only where it is more common.
Maybe he meant double clicking the top bar, which does seem to be common toggling maximise/restore down (I'm not sure of subtle differences between maximise and fullscreen)
Double clicking the top window border is like maximize, Alt-Enter full screen is in front of every thing else with no borders. Your right though sub could achieve similar results if the top menu was hidden when maximized, and the user configured the task bar to not show on top of applications.
I would prefer the double click ability, as my HTPC keyboard is usually put away Full mouse control of the interface would make things simpler. Even a right click full screen toggle would work.