Since it is a two sided platform, not a single product ("phone and apps"), and as Apple is constraining the payment downstream market, and causing harm to the consumer (30% markup), there is a case.

Some textbook rulings will apply, since Apple exces admit that if they do no constrain downstream, no one will use their 30% markup product. So consumers could be better off by definition

although Apple will argue that they will charge more to downstream producers directly, but this is also difficult to argue (they can do this anyway and extract rents from consumers and producers at the same time).

If you have a monopolistic platform market on which you also produce, it seems rather hard not to get sued