However, let’s face it, Valve isn’t about to just let Microsoft run a GamePass app on SteamOS either.
SteamOS is Linux (Arch). If Microsoft wanted to put GamePass on Linux, there's no reason it wouldn't run on a SteamDeck. Valve isn't trying to create the same sort of "walled garden" which you get with other consoles. I have several games loaded up on my SteamDeck which aren't from Steam. And I can even add them to the Steam interface. Microsoft software not running on the SteamDeck is entirely Microsoft's choice.
Yeah, in the Steam Deck interview with ign they even mentioned how Steam Deck can play games from other stores mentioning even Epic. Any absence or ease of use issues is due to the other companies not bothering to support Linux themselves and requiring the community to figure things out with an example being heroic launcher.
Basically the writer is an idiot. As Valve has said repeatedly it's a PC... Embarrassing prompt from a site with pcgame in it.