No, he's talking about the compositor which has several gaming related features which then runs the Big Picture version of Steam for UI to select and manage games.
They're right that the compositor is gamescope, it's made by Valve and it has game-related features, but the person 3 comment-levels above me did seem to be conflating gamescope with the Big Picture UI. You can absolutely use gamescope and not use Big Picture mode at all, lots of people using Wayland do so by wrapping their games in a call to gamescope.