Linux/Unix has been used as a base overwhelmingly in pretty much every new consumer OS for decades. Not to mention Microsoft certainly cuts deals with manufacturers when it comes to windows on portable devices (I think at one point they offered free licenses on devices with screen sizes under 8 inches).

The steam deck is 100% usable without leaving 'game mode' even a single time. Something that is genuinely impossible using Windows as a base. That's the important part