If Android did belong to someone else then there could be an argument that IAP fees are funding it and subsequently required, perhaps even under less-than-competitive circumstances.

But if the court allowed, Google would be paying that company $10s of billions for the prime real estate their software, search and services enjoy and it would then be a very weak argument that only IAP funds its development. Just like it is when Google earns $100s of billions in revenue from Android now.