Are you seriously blaming Steam for game developers setting the price of their games the same on all platforms? They're the ones pocketing that 18% difference by the way and that 18% difference is literally the selling point that is supposed to lure in game developers to the epic games store. There has never been a promise of cheaper games.

The "most favored nation" pricing you're complaining about being abusive refers to Steam key sales on third party platforms and that pricing exists for a blatantly obvious reason. How much of a percentage does Valve get from that sale? 0%. Absolutely nothing. The developer generated keys are free and Valve will still pay the distribution costs (storefront, downloads, multiplayer, etc) for you. If it was possible to sell a Steam key cheaper on another platform, then nobody would buy Steam keys from the Steam store anymore, which means their revenue would tank to zero, which in turn means they would have to cancel free steam key generation, duh. Valve is being extremely accommodating here and you're twisting it into its opposite, which is pretty disgusting.

The problem is the Steam Distribution Agreement isn't public so I don't have a way to prove if this is true or false.

At the same time there are articles like this https://www.bbc.co.uk/news/articles/cx2g1md0l23o on the bbc claiming a tribunal ruled a case can continue against Valve for "forcing game publishers to sign up to conditions which prevents them from selling their titles earlier or for less on rival platforms".

The website for the law firm clearly states the price parity is not just for steam keys https://steamyouoweus.co.uk/the-claim.

If there was no evidence it would be thrown out. But until it concludes who knows what the truth is.