I remember in an interview talking about the Steam Deck and its controls, GabeN said (paraphrased) "What we learned from the Steam Controller is there needs to be zero learning curve. Players want to pick it up and understand it immediately."
Given that ethos, it's not difficult to understand adopting KDE over Gnome. Most of Valve's customers are coming from Windows, and KDE resembles Windows' UI, where Gnome resembles iOS after a stroke.