It’s misleading, at best. They don’t actually restrict sales on other platforms at all. You’re free to sell your game at whatever price you want. The only restrictions they place are on Steam keys which unlock the game for a Steam account. They restrict the price of Steam keys, because they want price parity for Steam keys. But you’re still welcome to sell non-Steam versions of your game at whatever price you want. Hell, you can give it away for free if you want, as long as it’s not giving away steam keys.

For instance, GoG doesn’t distribute games via Steam keys, so you can sell your game on GoG for cheaper.