The study was done in 2004, which is more than 15 years ago. How about performance after 2005? I talk about annualized return, not Sharpe ratio.
Below is the annualized return of SPX and BXM
1988-2004 SPX: 12.4%, BXM: 12.7%
2005-2019(Apr) SPX: 8.7%, BXM: 5.3%