The author was trying to make the point that past injustices done to people of your own particular ethnicity are no justification for your present behavior and attitude.
In the history of the world, whether were labeled slaves or not, many millions of people of many different ethnicities have been de facto enslaved, imprisoned, or killed by others.
I can't go to Italy and start demanding reparations from Italians because some of my distant ancestors, Slavs, might have been taken as slaves by the Romans several thousand years earlier.