this movie really makes you realize how completely racialized is the American understanding of slavery
In much of the world for much of history where there was slavery, the slaves and the slave owners were of the same race. America is an exception to that. That seems to have warped Americans' understanding of slavery. It's hard for Americans to think of slavey as a unique evil by itself. Instead we view slavery as bad because in our experience of slavery it was racist. We mostly view American slavery as bad to the extent that it involved racial differences between the slaves and the slave owners.
This movie is a perfect example of that point of view on slavery. The people who made this movie as well as the viewers who watched it aren't bothered by the fact that it completely reverses the role of Dahomey in African slavery because the reality of slavery in Dahomey doesn't bother them that much. Blacks enslaving other blacks doesn't bother them that much because both the slaves and the slave owners were the same race.
A similar historical movie about America where America never had slaves and actually fought other countries to free slaves would never get made and if it did get made would be far more offensive to a general audience because of the history such a film would obscure. In the American view, racialized slavery is too great an evil to allow ourselves to obscure the fact of its existence in a movie. The same isn't true for non-racialized slavery.