I truly doubt this movie is going to further the absurd argument that "the war wasn't about slavery," since vampirism is used as a metaphor
for slavery. Using metaphor to stand in for something is a fine tradition in sf/f and as long as it's accurate, it's okay. And if the metaphor is needed so that modern audiences can understand the mindset of a distant period of history, the metaphor can actually be better than a straightforward approach to the same material.
Anti-slavery advocates before the war often described slavery as something that threatened the nation as a whole because of its corrupt influence and ability to sap the strength of a free-labor economy. I've never read anyone from 150 years ago use vampirism as a metaphor per se, but that might be just because it didn't become part of popular consciousness till Dracula
was published in 1897.
Vampires being far more popular now, they're a good way to translate the mindset of the time into terms a modern audience can understand and also be motivated to pay $12 at the box office to see.
If that's the idea, it's pretty ingenious.