Chattel slavery was legal between the 15th and 19th century, so slaves were uniquely accounted for accurately during that time. Now that slavery is officially illegal (unless the government has a good enough excuse), instead of legal, accounted for chattel slaves you get the de facto / black market / "technically-not-slavery" categories you describe and the job of estimating counts becomes more difficult.
Sure, "modern" slavery is more akin to historical/ancient slavery (i.e. ostensibly based on punishment, debt, or civil status), and yes 19th century Southern American slavery was particularly brutal, with cotton becoming the world's top cash crop on the economics of the cotton gin + slave labor.
But my point wasn't how many slaves there were between the 15th and 19th century, use any number you like, we all have some sense of how bad things were. My point was that there are tens of millions people living in slave-like conditions right now, while your point seems to be something like, "but it's modern slavery!"
Sure, "modern" slavery is more akin to historical/ancient slavery (i.e. ostensibly based on punishment, debt, or civil status), and yes 19th century Southern American slavery was particularly brutal, with cotton becoming the world's top cash crop on the economics of the cotton gin + slave labor.
But my point wasn't how many slaves there were between the 15th and 19th century, use any number you like, we all have some sense of how bad things were. My point was that there are tens of millions people living in slave-like conditions right now, while your point seems to be something like, "but it's modern slavery!"