IQ is highly correlated with both income and wealth so it seems like a fair comment. Of course not all billionaires have a high IQ, but far more billionaires do than your average person.
survivorship bias.
there is absolutely nothing stopping a poor child in sierra leone from becoming the next einstein, outside of access to things that should be considered mandatory for human life.
Not in the way you're implying. There's an IQ threshold that correlates somewhat with income, but correlated gains drop (kind of vanish) after it, and that threshold is pretty low; it might be 100.