While I'm sure that Valve would love to eat away at some mobile revenue, I don't think that's the goal here, at least not primarily. Mobile gaming (i,e, phone based) is largely a matter of convenience. Got some time to kill and you already have a phone anyway. Adding a $400 entry point is unlikely to be something that casual mobile gamers are going to do for the most part.
I think Valve has Epic and other PC storefronts in their sights with this. They want people to make their purchases on Steam, banking on the fact that most PC gamers already have a large Steam library, and they can install on multiple devices. While somebody could certainly pop Windows on this and add the Epic store after the fact, it's incredibly unlikely with the base model, which will almost certainly be the most popular option. I think they're trying to give people another reason to spend their money on Steam rather than elsewhere, by creating a new product type that lets them use their library in a new way.