It's true that Gamescope is a Wayland compositor, but it does not support Wayland clients ("native Wayland programs" if you're unfamiliar with *nix terminology). In handheld mode, everything you see on your screen is an X11 client running under Xwayland. I have no interest in arguing over whether X11 or Wayland is better (I've used both and both work fine for me) but I would not use the Steam Deck to argue that Wayland is being stress tested by hundreds of thousands of gamers when none of their games are using Wayland.