I think it is in fact not the case that high income people generally have significantly higher IQ than average. As for billionaires: when was the last billionaire IQ survey done?
https://www.sciencedirect.com/science/article/abs/pii/S01602...
https://www.sciencedirect.com/science/article/abs/pii/S01602...
https://www.sciencedirect.com/science/article/abs/pii/S01602...