I'm not quite getting your point.
The parent seems to be saying that when you take the difference between physical and digital goods into account, that Amazon is leaving him with a similar slice of the revenue.
You seem to be arguing that he has alternatives to Amazon.
However, you also seem to be making the point indirectly that the motivation for selling on Amazon, and why businesses sell in the App Stores, is that they want the additional sales that come from targeting those marketplaces.
Isn't then there little real distinction from selling products on Amazon (where you could sell elsewhere, but dramatically fewer would see and purchase your product) or the App Store (where you could make a web app and sell elsewhere, but dramatically fewer would see and purchase your product)?