Source is this video:
Windows Was The Problem All Along - Dave2D
We could obviously compare performance between windows and steamOS before on the steam deck, or between windows and Bazzite on other handhelds. But this is the first time we have had official windows and SteamOS builds for the same hardware.
Because DirectX is more than a graphics API.
https://en.m.wikipedia.org/wiki/DirectX
A fair amount of what used to make DirectX an everything API has been deprecated, but if you are already using Windows stuff for networking and audio, then you may as well use the graphics APIs too.