Only 400,000 came to the US. He is specifically talking about the US. The reason the population of Black people grew was from creolization or higher birth rates in the US in comparison to the other parts of the New World during slavery. The US had “breeding plantations” as well. In other parts of the New World importation occurred for a longer period of time. Sexual exploitation of slaves was a key tool of control in the US. Exploitation included forcing slaves to mate with each other as well as White exploitation of Black women and men. There existed the belief in the US that slaves who were creolized (born in the US) were less likely to rebel or try to escape as they knew no other way of life. This encouraged more “breeding,” i.e. sexual exploitation.