That figure of 13million doesn’t include the descendants of slaves. Since they were slaves too they should be included. As a percentage of global population, at the time, the figures of African chattel slavery would also be much higher.
Your comments about how modern slavery crosses racial lines also highlights another difference between the two. People aren’t enslaved because of their race. Africans were. These were human beings who were seen as property, not human beings. They were believed to be a sub species, lesser, simply because of the colour of their skin.
Modern slavery is a scourge that needs to be eradicated, yes. But that shouldn’t be used to belittle nor diminish what Africans went through. Both are crimes, it’s wrong to argue it’s worse today.