It’s based on devices. But, I wouldn’t consider it as diluting the Windows share. A user might have any combination of devices. Maybe they have PS5 as their home gaming device and Deck as their handheld device. They could also have Windows PC and Nintendo Switch. Or maybe they have Mac laptop and Linux desktop. I for one belong to the Linux desktop and Steam Deck camp. Steam Survey only tells how many Windows, Linux and Mac devices Steam users use, but, for example, not how many hours each type of device is used.