The arguments in this case are relatively subtle.
Of course Jefferson on slavery is still atrocious. But then we all knew that before this book came out. Did Jefferson change his views on slavery in the 1790s? A lot of historians dispute that in the new book. It's not a crazy idea. Susan Dunn's work comes to mind (she is in the historians' club and presumably is not greeted with the same skepticism as the author of this new book) as an example of the evolution of Madison's thought on a lot of these issues.
Anyway - the link above is an interesting read. I still have a hard time understanding what's genuinely newly revealed about any of these recent scuffles. I stand by my Jefferson/Heidegger anology as being apt.
So in case you were tempted: please don't look to either Jefferson or Heidegger as personal role models.