If you sell digital goods for use on an iOS device then you must also offer them via in-app sales, for the same price as you do elsewhere.
Since their app is only an iOS app, everyone will continue to buy via the in app purchases even if they did offer it online, and for each of those purchases they have to pay the Apple tax.
But this policy only applies to subscriptions, not to purchases. In particular:
>Publishers set the price and length of subscription (weekly, monthly, bi-monthly, quarterly, bi-yearly or yearly).
does not sound like it is in any way compatible with the book world's agency model.
Edit: found an Ars Technica article here:
but it seems that this policy was unclear even at the time according to the terms, and it's certainly not enforced with the Kindle app, so it's unclear what's going on here.
That's pretty much the problem in a nutshell - the policy's unclear, and it's always unclear until Apple brings the hammer down. Not a good environment in which to try to build a business.
Books are explicitly included in that document.