you don't necessarily need a monopoly to have a "dominant marketing position". being one of the two major OS's and losing access to 30-50% of a nigh-universal market because the its not in the interest of the platform's profits would definitely count as an anti-competitor move utilizing the company's weight. AKA anti-trust.
American teens are not Americans are not the rest of the world
Apple is an American company accountable to American laws and critiques
Also, the relevant market doesn't have to be "Americans." It's completely reasonable for it to be "teens in San Francisco" or the like, if that's the group of people you need a messaging app in order to interact with.