i think valve is over reaching with this policy.
It should be that you need to prove the art used in the game itself that does not breach any copyright. The tool used to create said art has no bearing on the final art with regards to copyright.
Otherwise, would valve also have mentioned that the developer should also produce evidence of their photoshop license/subscription (if they used photoshop in the course of making their game)? Do they need to check that the version of windows being used to make the game is a licensed one?