I still think it comes down to iOS and android being 2 fundamentally very different products.But for google to be found to be a monopoly when it offers the ability to install alternative app stores while Apple does not and Apple has been found to not be a monopoly is an inconsistent result. The law has not been applied equally to both parties.
With the iPhone, Apple managed to demonstrate during the court case that the App Store was in fact a key selling point of the iPhone. Customers buy the iPhone knowingly fully well that they have to purchase apps through the App Store, and in many cases, expressly preferred it that way. For them, the restrictions of the iOS App Store was a key selling point, not a drawback.
However, with Google, the problem was that customers were not buying phones from Google, but from other smartphone OEMs like Samsung. On one hand, you had Google claim that android was more open, and yet on the other hand, Google was effectively bribing developers to not bring their app stores to android, which goes against the very premise of their platform. So you can argue that customers were not getting what they paid for. They expected more choice in the form of say, more app stores, which never really happened because Google was working behind the scenes to keep it that way.
So the difference here was that Apple had been clear about their lock-in right from the start, while Google was sneakily trying to change the terms of the deal to limit competition. And especially under US antitrust law, because Apple was offering an integrated product that it fully controlled and that customers were fully aware of the implications, Apple was therefore free to set the price of entry however it chooses.
This means that yes, Apple is perfectly justified by law to charge developers 50% or even 70% if they do desire.
So the lawsuit is not about whether the companies are monopolies to not, but whether there was any misrepresentation to the customers.