I feel like it's worth pointing out that in the industry that spurred the development of anti-monopoly legislation, there were ~20 major companies. Or ~150, if you go by the definition of "large" established ~30 years later. (By either definition, there's 7 today in the same industry, if you're wondering).
> Surely by moving the goalposts you can make everything look like a monopoly.
Walmart doesn't have a monopoly on SAE 5w30 motor oil. You can't make it look like a monopoly when it isn't one, because when it isn't you can identify competitors who sell substitute products to the same customers.
> In the end both mobile platforms have practically the same popular apps.
The market they have a monopoly on is iOS app stores, not individual apps.
It's very straight forward. For Google Play to be in the same market you would have to be able to use it to install apps on your Apple iPhone. Since you can't, it isn't, and since there is only one app store that can, it's a monopoly.
Notice that it has nothing to do with the fact that Apple also makes the phones, outside of control over the phone being used to enforce the app store monopoly by locking out competitors. If Amazon for some reason had the only app store for Apple iOS devices, they would be the one with a monopoly in that market.