It's pretty disingenuous to compare numbers from two different time spans. I don't know where the authors numbers come from but he says the drop was 20% in 2008.
Your numbers add an additional 6 months during which the S&P 500 dropped significantly: https://www.google.com/finance?q=INDEXSP:.INX
additionaly, the time period you cite seems hand picked to start at the high and end at the low.