Well, it is about code quality. And the same codebase should work on different hardware, which is not something that is required in downstream forks.
But it is sad to see that the driver was submitted in the past, is still actively developed and improved, but there doesn't seem to be plans of submitting them again.
Also I don't think that a platform driver is so complicated that it requires such a long time for mainlineing. It not a filesystem or VPN.
Nothing of this is a burden, it is just part of being a good contributor that reads and follows the rules. Contributing is pretty easy, when you have read and are following the guides. If you haven't already, you should give it a try.
I am pretty sure that this isn't the first contribution of Valve to the Linux kernel. It sounds more to me like "works for me, don't care about others" attitude. Which is not a good attitude to have when working in any collaborative project. (Not necessarily against the developers, could also be management.)