My understanding is that game mode uses Wayland as the display server, and then game scope is run as a compositor for running games and apps. However dispite game scope running on Wayland, it doesn’t support Wayland clients yet. So all games running on it have to be run through xWayland instead.
Forcing a program or game to run in Wayland mode will prevent it from running, even though the base display server is Wayland. Honestly it’s a confusing setup, but I’m assuming backward x support was the priority for compatibility reasons.