That study is US-only, too. If you're making a solely US-based ad-servicing service (which makes very little sense, and if you are you should spend time on internationalization before spending time on your iOS app), otherwise if you are making a subscription service only in the US that does make sense.

And even if the trends did hold up internationally, and I really suspect they don't, Android would still be better for ads and subscriptions internationally due to nearly three times the market share, despite a bit more than half the disposable income.

I don’t see why the trends wouldn’t hold up internationally. Android is more popular than iOS internationally because the vast majority of the world has a lower PPP than the US. Android is popular in places like India which pale in comparison to US revenues per customer simply based on PPP alone.

I really don't believe that is true. Even in countries with high GDP iPhones can be much less popular than Android, for example Germany, or Italy, or Japan, or Finland, by pretty massive margins.

People don't really buy iPhones as soon as they have money unless it is a status symbol in their culture, which it really is the more the country is Americanized/anglophone. I think that correlation holds up much better than GDP PPP. There is no real reason for money to predict iOS market share except for that.