I would imagine it’s just some sort of really basic shim driver to get a communication channel between the controller and steam, then from there everything just goes through steam input.

That’s the kind of choice you make when you need to just get ‘it’ working. IIRC when the deck, its input depended (more?) on the steam client too. I’d imagine that they’ll finish the slow part of the process eventually to make the controller work more generally too.