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.