Google controls the API but Amazon is free to expand the functionality on their devices.
This way, the developer can simply call Context.getSystemService(CLOUD_SERVICE) and have access to whatever cloud service the user as installed.
Every other part of android, from keyboards to cameras, works this way and these services could as well.
Then, having an open source community contributing with open implementations could help filling the gaps to make life easier to those players (and probably lowering the bar to allow new ones to join) and, who knows, to make it possible to have an AOSP device fully compatible with Play Services APIs without the need for gapps, probably just using some private server or other custom solution. Maybe I'm misunderstanding something somewhere, but that seems to be the goal of this project. If it is, then I really hope for its success.
This project and some of Amazon's libraries basically take a subset of Google's Play Services API as canonical and reimplement those. That's not an ideal solution either, but at least it provides minimal pain for app developers.
The cloud service version and the client library code would not be coupled to OS version and updated with APKs just like how Google Play Services does. Its not any harder than exactly what Google is currently doing with play services.
I'm not proposing anything that isn't already being done besides some standardized calls and some extra service discovery.