The first sentence in your first link links to an announcement saying this:
> Android has always allowed people to get apps from multiple app stores. In fact, most Android devices ship with at least two app stores preinstalled, and consumers are able to install additional app stores. Each store is able to decide its own business model and consumer features. This openness means that even if a developer and Google do not agree on business terms the developer can still distribute on the Android platform. This is why Fortnite, for example, is available directly from Epic's store or from other app stores including Samsung's Galaxy App store.
The implication is that apps can choose to only publish in stores with more favorable IAP terms, not that each store is somehow eligible to a fixed percentage of the $120/yr fee for a costco membership that may or may not have originated within the app store's ecosystem. Otherwise, at a cut of 30% per store, an app would owe platforms 120% of subscription revenue to be on merely 4 stores, regardless of how the customer converted.