I think they are either refering to child labor in cobalt mines, which was used for Tesla power elements, or to his father's emerald mines. There's no evidence of direct people-owning slavery in either case, still a lot of shady and immoral (and probably illegal) practices to earn money in both of them.
I think they are either refering to child labor in cobalt mines, which was used for Tesla power elements, or to his father's emerald mines. There's no evidence of direct people-owning slavery in either case, still a lot of shady and immoral (and probably illegal) practices to earn money in both of them.