Which is deceptive, at best. Steam doesn’t have pricing clauses for developers’ games. The devs are free to sell their games anywhere they want, at whatever prices they want. But Steam does have pricing clauses for Steam keys. Basically, what allows you to register a game to your Steam account.
You can sell your game for whatever price you want, as long as it’s not the Steam version of the game. They don’t want you giving away Steam keys for cheaper than you can often buy them on Steam. And this makes sense; Steam has a vested interest in protecting their own game keys, and encouraging players to shop on a storefront that they know is reputable; Lots of steam key resellers are notoriously shady, for instance.
Basically, the dev can go sell it cheaper on GoG, or Epic, or their own storefront if they want. As long as they’re not selling Steam keys, they’re fine. But players like having games registered to their Steam accounts, because it puts everything in one place. So devs may feel shoehorned into selling Steam keys (which would invoke that pricing clause) instead of selling a separate version that isn’t registered to Steam. But that doesn’t mean Steam is preventing publishers from selling elsewhere, or controlling the prices on those third party sites. It just means Steam has market pull, and publishers know the game will sell better if it’s offered as a Steam key.