"Escape hatch" specifically refers to the speculation that Valve is positioning themselves in a way that they can't be forced into paying fees for existing on the Windows platform, and that if push comes to shove they can say they only support Linux now. This hasn't happened yet, but it's a strategic stance which will likely prevent it from even beginning to happen. This doesn't have to do with the Steam Deck specifically; it was also part of their intentions with the Steam Machine and etc.

Sometime around Windows 8 Microsoft started making noises about closing the Windows ecosystem and making people buy software through their store. This would have shut things like Steam out, so Valve said "Okay, we're going to make a Linux-based gaming platform, because we think gamers will follow us and not you. Also we're going to create console-like gaming PCs called Steam Machines and make our own controller, because we think we can win against Xbox, too." Microsoft didn't lock down the platform, Steam Machines didn't really go anywhere, but it laid a lot of the groundwork for the Steam Deck.