Well, slavery involved diversity, and a nation with "willing" slaves (as in, ones who obsequiously work and don't revolt ever, likely through "diverse" religious education indoctrinating them to never rebel while diversely indoctrinating their owners to never free them) will be more successful than a nation without them. So... That's technically a diversity-is-our-strength thing.
Well, slavery involved diversity, and a nation with "willing" slaves (as in, ones who obsequiously work and don't revolt ever, likely through "diverse" religious education indoctrinating them to never rebel while diversely indoctrinating their owners to never free them) will be more successful than a nation without them. So... That's technically a diversity-is-our-strength thing.