Then by all means check out the figures for yourself and then tell me that they make sense to you. 1860 Census
If this same professor stated that 78% of all black men today suffer from the effects of slavery on their ancestors would you be so quick to accept his statement? Or would you say, "Whoa, that can't be right" and dig into it yourself?
I should be surprised that wasn't closer to 100%, at least to some degree. There are things in my family history that color my attitudes about things in my own life; why should that be not true of anyone else? The only question is one of degree, and whether there is anyone currently living who is justly to blame.