I think what the original post was saying as that people complain about when minorities are placed in setting and argue that the reason they shouldn't be there is because of historical accuracy. While a white actor can be written into any setting and people will accept it.
Example: You can't have an African travel north and become a knight, but its okay with white boy being raised in a Native American tribe and then save the day
A recent Biography about Alexander Dumas' dad (titled The Black Count) is supposed to be made into a movie in the near future. If that actually happens, I am curious to see how the "historical accuracy" crowd reacts.