The size of the cut is what they use for the appeal to the public to build their social narrative, but legally/economically speaking it’s not really the problem. The problem is that Apple effectively forbids developers from having any other mechanism to transact with customers except through their marketplace where they take the 30% cut, hence the lawsuit being about monopolistic practices, not the amount they’re charging.

Valve handles things completely differently. Sure, listing on the Steam store requires giving Valve a 30% cut of the purchase price, but Steam doesn’t demand a 30% cut of any and all transactions that happen within or related to the game like Apple does. You also don’t have to buy a game from the Steam store to load it and launch it from the Steam client. And Proton works with a lot more games and applications than just those on the Steam store.

The fact that the two companies charge a similar price for a single relatively similar business case oversimplifies a lot of how the two companies operate.