If you define your market small enough anything can be a monopoly. I bet Apple has a 95% market share on phones that cost more than $1500. I bet Google has 75% market share on phones less than $100. So what?
Apple's control of the app store makes the $1,000 phone more appealing. If they opened up the iPhone to other random app makers, the phone would be less appealing and they wouldn't sell as much.
Do you think that for an App Store attached to a phone whether the phone cost $100 or $1,000 matters?
Because everyone related to those App Stores does.