FWIW it doesn’t seem like Valve is really looking to make Steam OS a general-purpose operating system. They are developing it to work on their hardware offerings and may or may not add support to other hardware. This means that they may not work on supporting Nvidia cards well unless they start shipping gabe cubes or decks with Nvidia hardware.

Just because a corporation is backing the OS doesn’t mean that they are doing so for anything beyond their own products.