Jack threw around the term slave, because for decades that was all they had been to him. Rightfully they were still people, like DeWitt pointed out, they still had minds of their own. It was not until Ianto that Jack saw the world changing, realizing, as he had as a young man in the camps, that slaves were humans, that he use to be human.
"Ah a girl." He smirked. "I bet she can keep you on your toes. I knew this girl once, feisty little thing, called me all sorts of names. They all didn't hurt but it was the fun of having that banter with someone. That was, oh lord," he laughed. "That was when I was still human."