The author's stock market example fails to talk about percentage returns and makes no sense to me. "Will a month of S&P 500 ever drop more than $1000? Using t

The author's stock market example fails to talk about percentage returns and makes no sense to me. "Will a month of S&P 500 ever drop more than $1000? Using the same procedure but with data from the S&P 500 index, the central limit theorem suggests that a drop of more than $1000, when looking at a time horizon of 30 days, happens about 0.9 % of the time."