I 100% see the Framework Desktop and Steam Machine less as competitors and more as separate flavors of the same form factor: Steam Machine for the lower priced, casual plug-n-play gaming use case, and Framework Desktop for the higher priced, professional use case.

Seeing as the Steam Machine feels like a way to get traction for SteamOS just as much as it is a venture for selling hardware, a partnership between them selling Framework laptops/desktops verified for SteamOS seems like exactly the type of thing Valve would want.