AFAICT, the article is saying that if there's a way to purchase content externally and move it onto the device, there must also be a way to purchase it internally with the AppleVAT.

So they're definitely targeting books purchased off device.

Let's hope so, but the article put it differently - if an app presents content bought elsewhere, there must be an option to buy that content also in-app. So if you are right, the article is wrong.

That's isn't to say that they won't try to find a way. :)