Apple has 60% of the mobile market in the US[1], and the App Store is responsible for more than 100% more revenue than the Play Store, too[2].
Regulatory bodies aren't concerned with layman definitions of monopoly[3]:
> 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.
[1] https://www.pcmag.com/news/ios-more-popular-in-japan-and-us-...
[2] https://www.businessofapps.com/data/app-revenues/
[3] https://www.ftc.gov/tips-advice/competition-guidance/guide-a...