I agree. But it is the only part of apples operations where they compete and have control over competitors.One could also argue that whatever Apple has inside their App Store cannot possibly ever be considered a monopoly as the Apple App Stores only exist on Apple devices which themselves have plenty of competition that sells in greater numbers.
If you start to divide it up by the store it quickly becomes ridiculous as Target would also have a monopoly inside Target Stores, Walmart would have one in their stores, even the local mom-and-pop grocery store has a monopoly in their tiny corner store in that case.
Of course Apple rules the Apple App store - just like any other store owner rules their own store.
But yes, I agree, I think even that is unlikely for the reasons you note.