In the Los Angeles Times Hector Tobar writes that "In "The Half Has Never Been Told," Baptist adds many new, stark and essential elements to that story. His most important achievement is to show us how the "dismal science" of economics served to make the lot of slaves even grimmer."
If Baptist were to do this it would be a nice trick. Thomas Carlyle was the one who named economics the dismal science. What did he find dismal about it? He thought it was dismal that economic theory did not provide support for slavery and that economists like John Stuart Mill supported emancipation.
I think there are a number of problems with Baptist's book, but I suspect this quote just reflects Tobar's ignorance.