Earlier quoted context omitted.
Gramatically you might be correct, but effectually ars is correct. You will not be allowed to access the content if Kindle does not allow you to buy books through IAP. That is exactly what ars is trying to communicate.
Explain how? Where is Apple saying you can't access content not purchased through them? All I see if Apple saying if the apps sends users someplace else to purchase, then the app should also allow them to purchase from inside the app. There is nothing about what content can and can't be accessed.
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.