It's simple. There exists a market for app stores on Android, there doesn't exist such a market on iOS. So, Apple can't be said to have a monopoly position in the iOS app distribution market, because, again, such a market doesn't exist, and there is no general obligation to create one (there is a different discussion about the app market, which Epic was attacking and which failed for now).
But on Android, you do have a market for app stores - there is Google, and then there are various bit players (F-Droid, Samsung Store, Amazon Store, and others). And Google is by far the biggest, and using their position to set the rules for all the others, including actively hostile actions like de-listing some apps if they don't offer exclusivity to Google Play, disallowing Google Play installation if the OEM doesn't ship it by default, etc.
But on Android, you do have a market for app stores - there is Google, and then there are various bit players (F-Droid, Samsung Store, Amazon Store, and others). And Google is by far the biggest, and using their position to set the rules for all the others, including actively hostile actions like de-listing some apps if they don't offer exclusivity to Google Play, disallowing Google Play installation if the OEM doesn't ship it by default, etc.