In 1860 there were 3,953,761 slaves counted in the census.
In 2016, if you count everyone under probation and parole in addition to those in jail and prison, the number is 6,613,500 (https://www.bjs.gov/content/pub/pdf/cpus16.pdf)
If you assume 40% of them are black, like the article claims, the number comes out to 2,645,400. This is considerably less than the number of slaves in 1860, or 1850 for that matter.
If you have a source to back up your claim then you should post it. I'm actually curious what sources the author might have been using.
I don't think you're acting in bad faith, I think you just read that huffpost article and assumed it was correct. I just wanted to reply since you were curious about what you might have done wrong.