History proves humans as capital exists. Think antebellum US as one example. Slavery exists in capitalism. There. Obvious. Thanks.
Just because humans used as capital does not mean capitalism should be associated with slavery - that's like blaming money/greed for slavery, should we fear money/greed? Of course not money/greed is normal, nobody want to work for free.
How about the cattle industry, because humans enslave cows/chickens/etc should animal lovers now blame capitalism for that? Obviously not.
Capitalism is the FREE exchange between buyer/seller and the FREE exchange of work for pay - in cases where one is a slave or one has no choice but to buy/sell, that is not the intended function of capitalism, instead it is exploitation.
So, as to your examples of slavery existing because of capitalism - you are wrong because slavery exists independent of capitalism, not because of capitalism, history shows that slaves trades exits whether slaves are bought/sold/traded or not or just forced to join armies to fight or forced to build pyramids.
Capitalism, the intended easy without force, eliminates slavery by allowing the worker to negotiate their wages or not work, without capitalism as the tool to negotiate pay - all we have left to convince others to work is force/convincing/extortion/begging/charity/etc, but without capitalism there is no exchange of work for pay.
There. Obvious. Thanks.