"We are now requiring that if an app offers customers the ability to purchase books outside of the app, that the same option is also available to customers from within the app with in-app purchase."
The only way you can get access to premium content on the Kindle is through PURCHASING. Apple is implying that you can only access content that has the capacity to be purchased in iOS.
This does not mean the content has to be purchased through Apple's in-app payments. It only means that it has to be purchaseable.
That is, if an app can consume premium content, that content must be purchaseable through the app. Any individual piece of the content can be purchased through the app, or elsewhere.
The only way you can get access to premium content on the Kindle is through PURCHASING. Apple is implying that you can only access content that has the capacity to be purchased in iOS.