Either you misunderstood IDGA's letter or you misunderstood gacba's comment, or both. None of this has anything to do with the 30% cut they take, so I'm not sure why you're mentioning that.
The complaint is in regards to Amazon controlling the cost of the app rather than the developer controlling the cost of the app. That's all. Gacba was simply saying that if you don't want Amazon controlling the cost of your app, then make it free and sell in-app purchases. Since Amazon doesn't control how much you sell your in-app purchases for, they lose total control over the cost of your app and the developer gains the control back.
The complaint is in regards to Amazon controlling the cost of the app rather than the developer controlling the cost of the app. That's all. Gacba was simply saying that if you don't want Amazon controlling the cost of your app, then make it free and sell in-app purchases. Since Amazon doesn't control how much you sell your in-app purchases for, they lose total control over the cost of your app and the developer gains the control back.