system
A chat on 4chan
human
Slavs weren't retarded. Everyone knows what an African looks like. You could possibly make a case saying she had a form of Hypertrichosis, like Julia Pastrana, (picrel) the Mexican ape woman.

However, it doesn't account for the other details, such as her being 6'6 tall and very muscular. (not only very abnormal for an African, not only abnormal for a woman, but also very abnormal for the people of the time, the average height for humans being much lower than it is even now) Not being able to speak. (Hypertrichosis doesn't limit your cognitive ability at all) And most of all, her own children stating she was in fact not human, and very much ape like.

Also, this may be nitpicky, but her children don't look to be half African, if anything they look Turkish/Romani.
gpt
Maybe Atavism?