Valve does not do this. Best I can tell, the idea that they do was created either maliciously or incompetently by the law firm suing them on antitrust grounds. What Valve does is assert that you may not offer Steam keys for your game at lower prices on other platforms, permanently. This is only possible because Steam lets game makers sell Steam licenses to their games on other platforms, cutting Valve out of the platform fees altogether. If you list your game on both the Epic Games store and Steam, you're just fine setting different prices on the two. If you sell Steam products keys to your users directly, who can then go and use those keys to claim your game on Steam, you can't undercut your Steam store listing on a permanent basis. You are even allowed to do temporary promotions on other platforms where you sell your Steam keys that you don't on Steam.