I as a consumer am free to choose alternative devices, and I have. A
vendor is constrained to go where the money is, and on mobile devices, Apple serves as the sole gatekeeper to a lot of money. And the vendors pay a usurious 30% cut to Apple
because there is no alternative. And high prices like that are
absolutely indicative of a monopoly. Do you think Hey would still be in the app store if Basecamp didn't raise a huge stink and Apple didn't have a pending antitrust hearing? For a monetization strategy that's identical to Netflix? If alternative processors were allowed for in-app purchases is 30% really the natural commission they would all charge? If it were allowed to say, "hey, if you subscribe through the website it'll be 25% cheaper because we don't have to pay the App Store commission", how many people do you think would take the cheaper option?
We can argue about technical definitions of what constitutes monopoly until the cows come home. But look at the behavior of vendors and the knock-on detrimental effects on consumers (high prices, lack of choice): these are the effects of monopoly, and the harms that that antitrust legislation is meant to prevent.
Stripe is successful without being a monopoly. Nokia phones had the market share to be called a monopoly, but they didn't wield monopoly power. Shimano is absolutely dominant in bicycle drivetrains, but if they tried anything from the monopolist's playbook bicycle vendors would laugh at them and SRAM would eat their lunch.