Earlier quoted context omitted.
Apple has a monopoly on iOS devices. An iApp only runs on iOS, so it's irrelevant that Apple is a minority of the mobile market because that's not the market being analyzed for the antitrust issues.
This logic makes no sense. Using your reasoning Nintendo is a monopoly because only Nintendo devices will run Nintendo games.
Monopolies aren't inherently illegal. See e.g., patents and copyrights, which are legally granted or protected monopolies.
What is illegal is anti-competitive practices. This doesn't require a dominant or even majority market position--authorities in the US and EU have gone after companies with relatively small market shares that engaged in anti competitive actions (though they haven't done so in many years).