I meant the opposite of that. The single asset that they own is less diversified and riskier than the S&P500, and therefore you can't take the annualized return

I meant the opposite of that. The single asset that they own is less diversified and riskier than the S&P500, and therefore you can't take the annualized return of the S&P500 and then conclude that "money makes money" on a founder paper-billionaire's wealth. The S&P500 is a portfolio that puts together all of these companies and diversifies them. In other words, Bezos is one part of the S&P500.