Amazon customers are still available on other platforms. They use credit cards and they can put their details into any site, and I imagine most of them frequently do. Their shipping address is also accepted by every company.
iPhone users don't carry a second Android phone, and their purchasing decision has committed them to only buying on the App Store for at least a few years at a time. And the crazy part is they are paying the Apple Tax even for services like Spotify that they might primarily consume on other devices. You can't make a store that ships to Apple users - only Apple can.
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)?
Companies like Amazon and Apple are free to set prices, but there are rules about being a monopoly and what that entails. Amazon is not a monopoly, it just has a dominant position. Apple has used technological means to make itself a monopoly.
Web apps are not a real alternative. Firstly, an app you can only use on a desktop is a non starter for almost every use case. So you need a mobile layout. Now, some features like background audio and video are not available as a web app. Some are less reliable like user sessions, timers, push notifications and offline behaviour. Technical innovation is not possible due to the standards based approach - for video calls you have to use WebRTC for example, for games you have to use WebGL. Some features like notifications, vibration, were delayed by Apple until users were trained to only accept native apps. There's others like battery status, Apple Watch, Settings pane that I don't know the exact status, but I'm sure App Store gets an advantage there too.
iPhone users don't carry a second Android phone, and their purchasing decision has committed them to only buying on the App Store for at least a few years at a time. And the crazy part is they are paying the Apple Tax even for services like Spotify that they might primarily consume on other devices. You can't make a store that ships to Apple users - only Apple can.