The idea is that you should consider the area around the notch to be extra, and not part of the normal screen. Show your game on the rectangular part of the screen, and use the extras for stuff that doesn't need the whole screen, like time, date, notification count, menu, whatever.
Having just the rectangular screen means not having the option of making those things visible, or having a smaller screen.
iPhones have the same problem. Ever played a fullscreen game and had part of the UI cut off into the notch or island. Yep, it's a problem.