@Seihanda I think the fact has either escaped the author/illustrator, or he didn't deem it to be too important to change up the envisioned design, since Dumas was still 3/4 white and didn't look typically black.
Either way changes to the appearance of historical figures was never something special in Fate anyway, so I don't know what's the deal with this one that makes it more baffling than Nero, Jing Ke, Napoleon, Caesar or Vlad for example.