Respectfully, I'm gonna have to disagree with you there. I've added quite a few desktop applications to steam on my steam deck, discord, chrome, Firefox, and they all take up the whole screen from game mode. I may have seen them running with black bars, but never double letter boxed like this.
more often than not, game mode will merely scale the desktop window size without maximizing it unless the window already is or is instructed to do so. this is how game mode's compositor works. it does not on its own force a change of aspect ratio. one big example of this is the Battle.net launcher's default size put the Play button just off screen, and the window looked ultra wide. resizing the window or maximizing it in desktop mode and returning to game mode made the button visible