Valve's better than Epic Games, but they still have every incentive to keep users in their ecosystem with DRM. To that end, the Steam Deck's Proton compatibility layer and touchpad driver are integrated into Steam, rather than functioning independently of it.
Using the Steam Deck is possible without buying any games on Steam, but making Steam the core part of the distribution inherently encourages its usage.
Edit: I like the Steam Deck, but I don't think it's unreasonable to point out the aspects of the platform designed to encourage the use of Steam.