The beginning demonstrates several hints that the plantation is is set in the modern day and isn't real:
In the opening, Jasper shoots a slave in the head with a modern looking pistol, as opposed to the musket he would have used if he was actually in the civil war era. The slave he shoots is also wearing a nose ring.
Daniel’s friend refers to him as “compadre” at the soldiers’ dinner. This modern Spanish word certainly wouldn’t be said by a random American Confederate soldier.
Veronica and Julia, as well as the other slaves, don’t speak in the black vernacular that slaves were known to speak with. Due to many slaves being illiterate, speaking with perfect English wouldn’t be likely, especially not modern English.
Eli stands face to face with Jasper in a challenging motion when his wife is brought up. Pretty much any slave that challenged his owner was almost immediately met with punishment of some kind (likely whipping). Instead, Eli is sent to clean the ashes of his dead wife, which is a pretty unnecessarily elaborate punishment until you realize that it's to break his morale, since he's not truly a slave.
The soldiers chanting "Blood and Soil" which is not a Confederate saying but rather one from Nazi Germany, also known as 'Blut und Boden'. This referred to Nazi Germany's ideal of "racially pure" citizens (the blood) in a location (the soil), essentially another way of saying "ethnostate". This makes sense as the men and women willingly participating in Antebellum are white supremacist neo-Nazis.
When she gets fed up, Veronica walks away from the cotton picking. This most definitely would not fly with the owners if she were a real slave, but since the cotton picking is just for the sake of aesthetic, they don't really need her to pick it, just for her to keep quiet and submissive about the happenings of the place.
Why is “Him” virtually never referred to by name? Well, the slaves are demeaned by having their names changed, and Eden/Veronica reclaims hers when she escapes. “Him,” being such a scumbag, is not respected enough by the narrative to warrant his name being spoken.
Why is there a shot of cotton being burned? Since the plantation isn't real, the cotton isn't being used, so burning it is just a way to be rid of it.
The Theme Park Version of the plantation seems more like it’s going for Artistic License – History rather than being wholly accurate, but with the realization that it takes place in the modern day, the inaccuracies make sense since no one at the park, whether they play a soldier or were forced to be a slave, was actually present during the Civil War, so they go by the most generic version.
Also, since the place is run by white supremacists, the entire Confederate camp is really styled after an idealized version of the real thing.
Many have noted that the soldiers and slave owners are all very blatantly evil, since they don’t really seem to use their disposable workers for monetary gain, which was the purpose of slavery. The movie has it be used to display a bunch of racists who use the park For the Evulz. However, that’s kind of the point. Slavery is no longer a necessary thing for America to function, at least, not in the manner that slavery was performed in Antebellum America; farmers can use tractors to do the same job as slaves but faster and more efficiently, and domestic servants could do paid housework. Modern white supremacists don't use racism to be useful or efficient, but as a display of irrational and unnecessary revenge and power. The park is chock full of such sadistic racists who happen to be blindly nostalgic of such a horrifying era because that's precisely whom it caters to!
The caller who woke up Denton at 3 a.m. was most likely Elizabeth. He rants a bit about Veronica's husband and daughter looking for her; his daughter was likely complaining to him about all the publicity his high-profile captive's disappearance was attracting. This would make it satisfyingly appropriate that Elizabeth was the one who alerted Veronica to her means of escape.
