Valve bans developers from selling their games cheaper on other platforms

I’ve just checked their steamworks partner documentation. This only applies to steam keys.
partner.steamgames.com/doc/features/keys

You should use Steam Keys to sell your game on other stores in a similar way to how you sell your game on Steam. It is important that you don’t give Steam customers a worse deal than Steam Key purchasers.
(…)
It’s OK to run a discount for Steam Keys on different stores at different times as long as you plan to give a comparable offer to Steam customers within a reasonable amount of time.

That, imho, is quite reasonable, because Valve provides these keys to the developer free of charge, i.e. the developer does not have to pay 30% commission on these keys.

Developers are free to sell their games on other storefronts (like Epic or GOG) for a lower price than they do on steam, permanently.