Valve actually don't forbid you to apply DLC distributed outside of their platform to apply to game you bought on steam though. There are tons of visual novel game distributed this way. Main game on valve and adult DLC on dlsite...etc only.

Besides that, almost all online game except for a few provide item though the in game store. I don't think they even need to pay for a cent to valve unless they want to use steam checkout service.

I wonder who ever used Steam would even buy this argument. It looks totally non-sense to me.