They were using indigo in ancient times, people been knew about indigo dye. Indigo was a chief crop in the south because it was mostly sourced from India worldwide until it was able to be synthesized. So it was a cash crop, now yeah slaves would've worn denim because it was workwear along with burlap.It would be more accurate to say that they created the process of separating the dye from the plant.
Denim, as recognized today was...if I'm not mistaken...from France in a town called Nimes. Hence, de Nimes, or "from Nimes". That eventually became denim as we know it now.
Your video is true and I'm not trying to take anything away from it. Just a bit more clarity.
Off topic but somewhat on when they mentioned Levi's.....Brooks Brothers started out making plantation clothing for slaves but has hidden it from company history. They make no mention of it EVER which is some real whitewashed history.