What Apple actually said (emphasis mine): "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 user…
That is exactly the way the Kindle app on the iPhone works. When you click "Get Books" within the app you are redirected to Amazon's website on Mobile Safari where you can purchase the book then return to your app.
With that in mind I don't see how what you are suggesting is any different than what Ars is saying.