You can only judge people by the standards of their day. Dempsey likely held the beliefs that lots of people did in America over 100 years ago. People today like to think they would be the one who's different and bucks against the trend if they lived in previous eras. But they probably wouldn't have been.
No disrespect to Dempsey, but Frazier is just the upgraded model. Dempsey < Marciano < Frazier < Tyson
While I agree.... Dempsey wasn't racist himself. Even his most adamant critics concede that. He had black sparring partners, and black friends and became excellent friends with Joe Louis later in life. But yes you hit the buck on the head. Abe Lincoln would ironically be viewed as a racist today.
Which historians, and what years did they make the claim? His hook was certainly renowned in its day and for a long time afterward.