It has nothing to do with App Store or operating system, that’s not even relevant. Here’s what is:
You can’t define the market so narrowly that it only includes a single brand, then claim that brand has a monopoly.
The relevant market isn’t app stores for iPhones, the relevant market is app stores for smartphones.
But even though Apple doesn’t have a monopoly, there are still rules. If Apple is in violation, they’ll be set right by the court.
An iOS app is fundamentally different from an Android app and it cannot run on Android.
How can you claim the same market when what's sold are not interchangeable substitutes?