The Linux Foundation is a 501c6 non-profit trade association. Their purpose is to help their members use Linux to increase profits. The promotion of desktop Linux, is not a priority of the Foundation.

The steam deck will use Arch Linux because it is cheaper to use linux than it is to pay licensing fees for a proprietary OS. Using a high quality rolling release Linux is also cheaper than writing one's own OS. In this regard, Valve is standing on  the shoulders of the devs who have put decades of work into making Arch what it is today. The Arch community owes absolutely nothing to Valve; and without Linux, Valve wouldn't have a product to sell.