He means the modern form of Western Slavery stems from the Roman decress in the 1400s and 1500s.
Supposed Islamic slavery didn't result in the death of American Natives or the forced relocation of peoples and the destruction of their heritage.
Additionally, the supposed 'slavery' that is mentioned in al-Quran al-karim is Highly regulated and reserved for certain conditions and NOTHING like colonial 'Western'/American/Roman slavery.
I would know... I am multi-national Muslim that has had servants and they are treated much better than how regular American minorities are treated. And their names and culture were not forcibly changed.
Sure, tell that to the Yazidi