It's not at all. I said potentially, Not a certainty.

People have forgotten or not read about the Windows 8 battle: https://www.pcworld.com/article/394953/why-does-the-steam-deck-run-linux-blame-windows.html

The Steam Deck and the software inside of it are the culmination of a nearly decade-long “hedging strategy” embarked upon by Valve chief Gabe Newell and company many moons ago, when Microsoft tried exerting more control over developers with Windows 8.

Valve is very fearful of that situation where Windows Store is the only way to buy games on Windows OS.

Overall Valve isn't going to push for Linux any further than Windows is going to push for a world where games can only be purchased through Windows Store. That at one point was a real fear and still somewhat is at Valve. Microsoft is positioning itself with GamePass to make the Windows Store more and more attractive. Even WSL can only be activated by visiting the Windows Store to download Ubuntu. While Valve is in the Embrace and Extend part, Windows is in the Extend and Extingish part. Valve of course, isn't going to do anything insane like "You can only use Steam OS to play these games." Yet. But they are going to push for Steam OS and potentially down the road only have features in Steam OS that they won't allow with Vanilla Linux or Windows. Imagine if Steam Link Mobile Streaming only worked well with Steam OS desktops because of proprietary kernel modules.