I mean let's be honest here, it's a pretty clear-cut violation of the rules. We all know the App Store rules can be ambiguous and loosely enforced, but this one seems pretty obvious:
The app put Amazon in a wrapper, injected code into the web viewer experience for the functionality, and then was on the App Store as a separate app. We all know if someone made an "Apple Shop Helper" app that showed, say, competing prices elsewhere, that Apple would shut it down in a heartbeat for displaying content from a 3rd party that did not consent to the integration.