"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."
What ars interprets that to mean (emphasis mine):
"If an app lets users access content that they purchased via Amazon's website, for example, then that same app must also let users buy the same book via Apple's own in-app purchase system"
That's not at all what Apple said. Apple said if the app lets you buy books outside of the app, i.e. you click the buy button inside the app and are then sent to the webpage to purchase the book, then the app must also let you purchase the book without leaving the app by using in-app purchases. Apple says nothing about not letting users access content they paid for from other sources.