but a bunch of lily white british dudes speaking shakespearean english is a-ok
That does bring to mind an interesting point. Shakespeare's
Julius Caesar was loaded with anachronisms. In fact, all of Shakespeare's works were. Despite being mostly set in various far flung exotic locales, all of his works described stuff like clothing as if it was contemporary English dress. A person portraying soldier under julius Caesar would act on stage wearing contemporary armor. The very idea that the portrayal of a fictionalized historical setting needs to be done in an "historically accurate" manner is a modern concept.
Not that modern movies and stories are any better at historical accuracy. When I was in college studying history, one of my teachers turned her Medieval History class into a film class. We spent the entire quarter watching Hollywood movies set in historical settings, and going over how accurate they were. Almost all of them were awful at historical accuracy. Some were glaringly bad (Gladiator), others were at least okay (Monty Python and the Holy Grail).
But the reality is that whenever we create a story set in a historical setting, it is by its very nature a complete fantasy peppered with anachronism and modern ideas. We are modern people; we don't live in these settings. Ultimately, we are being selective about what we add to that setting, whether we realize it or not. To create true historical accuracy, we would have to find some way to create something without human control. After all, history was not created, it happened.
When someone creates something, their intentions are carried over to the creation. These can be intentional or unintentional, but they are there. However, it is better for there to be a good reason to add in any element to a setting. To be honest, I have no problem with anachronism in historical settings as long as the work doesn't go around touting its "historical accuracy". As someone who sat through that history/film class I mentioned, I'm sick to death of bullshit claims to "historical accuracy". I'd take a setting rife with fantasy and anachronism as long as it puts forward good ideas.
And I don't think celebrating racism and whitewashing history to appeal to people's modern misunderstanding of race counts as a good idea, accurate or not. Of all the various things to focus on as important to historical accuracy, why choose racism? Why is racism one of the important and inviolable pillars of human society that most be consistently preserved in any and all works set in historical time-periods? People don't care that if they throw multi-color printing presses into ancient Rome (I'm looking at you Gladiator...), so why should they care about the accurate portrayal of racism?