Chattel slavery adds certain people, namely those owned as slaves, to the "means of production" column. It's not incompatible in any way with capitalism.
The fact that slaves are not free means that exchanges couldn't possibly be voluntary.
"Slavery is capitalism" is most certainly wrong though.
"Capitalism encourages slavery in plantation economies" would be a better argument, but might also be refuted given the gradual abolishing of slavery despite the continuance of capitalism.