That’s not entirely true. Valve forces devs to not sell Steam keys lower on other sites without also going on sale on Steam in a reasonable close amount of time.
I know it sounds the same at first, but it’s a drastic difference. You can generate as many Steam keys as you like and sell then on other sites, Valve won’t see a single cent from these sales. They however still provide their online services and servers for free for all those keys sold on other sites. It is quite reasonable that they force you to match prices since they literally are losing money (albeit not much) if you sell on other platforms. And I don’t mean lost sales, but infrastructure cost.
And additionally is this rule pretty much never enforced. AAA studios have special deals and indi devs aren’t worth the hassle.