“There is nothing anticompetitive about charging a commission for others to use one’s service,” Apple said in the filing. What about forcing someone or some company to use a service (apple's payment systems)?
No one is forced to use it: just stop developing for iOS. Same goes for any other platform like Google Play, Xbox, Nintendo, etc. Or selling concert tickets through Ticketmaster.
Also, it's not only about the App Store. You can't simply tell the iOS users of your mobile web app to use an Android device.