The case is going to not about that. The case is going to be about Apple not allowing developers to tell their users that they can buy subscriptions and in-app goods for cheaper through alternative payment mechanisms where Apple cannot take a cut. Payments is what is this is about (also see the FB release today).
Apple doesn't have to sell anything it doesn't want to but what it's doing is also preventing apps from telling users where they can buy these goods outside of Apple store unless Apple is offered a cut.
This behavior has a high chance of failing on anti-trust if it goes to a jury trial which is what Epic is trying to force (and likely others like Facebook will join or also sue) because Apple store is the only way people can install apps on iOS.