The claim is not true. The official rules are not forcing price parity.

You can sell on steam for 40$ and on gog or itch for 20$.

The only rule is that you want to sell a steamkey, making the game available through the service, to people buying from a different platform, you can't give out the steam key for cheaper on that different platform than steam customers can buy it on steam. You don't even have to pay steam the 30% cut if you're selling somewhere else.

You can even do temporary deal on a different platform, if you're doing a similar deal on steam "within a reasonable time".

https://partner.steamgames.com/doc/features/keys#2

And also, you are not FORCED to sell on steam. You can just not use the platform.