A number of historical television series feature racially diverse casts playing characters that were in reality white. So why do that?
Hamilton creator Lin Manuel Miranda has said that he used a racially diverse cast because it evolved out of seeing political debates during the time of the founding fathers were like rap battles. Other shows like The Great or Bridgerton do similar things with diverse casts.
Dickinson also does it, but has to play a delicate balance as it is set during the US Civil War with the storyline not ignoring the setting.
So the casting isn’t historically accurate, but it’s not even trying to. What it’s doing is subverting race.
It’s an act to promote racial justice. Racial inequality is like a cycle and the best way to break that cycle is to simply break the wheel.