The question addressed by the court is regarding who is the cause of any overcharging to the customer. The opinion argues that because Apple is exclusively dealing with the customer, it is able to sell on whatever terms it wants to the customer, so any overcharge is their fault. The dissent argues that because the developer has the power to dictate the price of the app on the store, any overcharge the customer pays is directly caused by the developer, because the developer could have chosen to sell the app at a fair price.
It is true that the ruling mentions that developers may sue apple, but that was never in question. Both the opinion and dissent agree on that fact.