I know how Valve’s publisher API works, others are similar in case you didn’t know. But that is only true for games that need online validation of some sort, DLCs for offline games don’t need to implement this.
Valve is hosting the game, providing the storefront and bringing in a lot of customers. If you didn’t think those 30% were worth it you would not have put your game on steam.
Plus all of this is irrelevant to the point that Valve doesn’t enforce price parity.