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.