Valve only requires that when you are selling Steam Keys outside of Steam. If you are selling the game as a standalone separately there is no MFN clause.
To expand on that, what's happening is that Valve allows developers to give a Steam copy of the game to anyone who bought the game on a different marketplace. Neither the user nor the developer pay Valve anything for that copy of the game.
Valve will only provide that free Steam copy if the price the user paid is equal to or greater than the price on Steam. This ensures that you can't effectively just sell Steam copies of the game at a lower price than on Steam itself.
That seems more than fair to me. Valve is providing a free promotional item and it has a straightforward limitation designed to prevent abuse.
