iPhone market share is very close to 50% in the US. You can't have a successful app launch in the US without making your app available on the iPhone. Apples policies affect the entire market, especially in the US.
If they're not already a monopoly in the US, then they might as well be since they have the same market control as a monopoly player. Also, a strict majority market share is not required for a company to be considered a monopoly. It's up to the courts to decide. [1] [2] [3]
[1] https://www.ftc.gov/tips-advice/competition-guidance/guide-a...
> Courts do not require a literal monopoly before applying rules for single firm conduct; that term is used as shorthand for a firm with significant and durable market power — that is, the long term ability to raise price or exclude competitors.
[2] https://www.justice.gov/atr/competition-and-monopoly-single-...
> In determining whether a competitor possesses monopoly power in a relevant market, courts typically begin by looking at the firm's market share.(18) Although the courts "have not yet identified a precise level at which monopoly power will be inferred," they have demanded a dominant market share.
[3] https://www.theverge.com/2019/5/14/18618127/apple-pepper-sup...
> The plaintiffs, meanwhile, will argue that these alternatives don’t matter. “The fact that they have a [less than] 50 percent market share of smartphones doesn’t mean they don’t have a 100 percent share of the distribution of iPhone apps — which they absolutely do,” says Rifkin.