What you're saying doesn't make any sense. Gamescope is essentially a XWayland-only compositor (with some basic xdg-shell support that's disabled by default). All games on SteamOS go through XWayland.

It doesn't yet mean that it suffers from the latency measured in the article, as the problem could very well be in something else, such as how KWin integrates with XWayland or how GPU drivers interact with it (especially that Nvidia drivers have a history of making XWayland suffer).

gamescope absolutely can have Xwayland child panes & this is currently the default on SteamOS running games with Proton, unless PROTON_ENABLE_WAYLAND=1 is set in the game options.