> 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. That is how that term is used here: a "monopolist" is a firm with significant and durable market power.
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> Apple phones make up only 13% of the phone market.
iOS has 60% of the market in the US[2].
[1] https://www.ftc.gov/tips-advice/competition-guidance/guide-a...
Also it's market-specific. As the other commenter noted, the market share is region-specific and in the EU higher than 13%. In the U.S. it's even higher at ~47% (https://www.statista.com/statistics/236550/percentage-of-us-...).
You could argue that users technically have a choice to switch to Android but due to network effect (e.g. your family/friends all use iOS) and lock-in features like iMessage and Facetime, it's often a false choice.
> Captive markets are markets where the potential consumers face a severely limited number of competitive suppliers; their only choices are to purchase what is available or to make no purchase at all
Certainly doesn't help that it's the current 30% but it being 15% won't change the fundamental issues being argued - whether Apple is abusing its market power and shutting competitors out of or subjecting them to unfair practices in its iOS ecosystem. E.g. unfairly favoring its own Music app.
I think it's a fairly straightforward case similar to Microsoft's antitrust case for bundling the Internet Explorer in its Windows OS a couple of decades ago. The argument then was that Microsoft favored its own browser and crowded out viable competitors like Netscape. If anything, the case is more obvious here since you can't even install any other music streaming alternatives on your iPhone if it's not in the App Store - in Windows you could download and install another browser (even if most users didn't).