I didn't say you couldn't. I said installing Windows is a complete mess on Steam Deck, which is true.
Valve has not fixed any of the issues, and why would they, the truth is that Valve doesn't want you to use Windows and other game stores on Steam Deck. Steam Deck "supports" installing Windows kind of like Macbooks "support" installing Windows. In truth, they don't really want you to do that.
The moment I wanted to install my GoG games for example, I needed a 3rd party launcher, on top of the GoG launcher, which was a disaster. And like I said, installing Windows on Steam Deck is not a solution due to all the issues, like LTT points out.
I know my GoG library works flawlessly on Windows, and Asus fully supports it and even made it part of their launcher.