You are absolutely right that the largest single day drawdown for the S&P was about 20%.

But my message is that we should not be sitting ducks for these extreme risks. There is no known method to accurately know the exact maximum drawdown that a security can get in 1 day.

Days aside. The drawdowns can be worse than we can imagine over several years. For instance people that bought the the S&P during 1929 broke even at around 1955. No profit, just breaking even. A time span of about 26 years [0]

[0]: https://www.macrotrends.net/2324/sp-500-historical-chart-dat...