1a : a source of supply or support : an available means —usually used in plural
1b : a natural source of wealth or revenue —often used in plural
1c : a natural feature or phenomenon that enhances the quality of human life
1d : computable wealth —usually used in plural
1e : a source of information or expertise
2 : something to which one has recourse in difficulty : EXPEDIENT
3 : a possibility of relief or recovery
4 : a means of spending one's leisure time
5 : an ability to meet and handle a situation : RESOURCEFULNESS
I'd say humans are resources under definitions 1a, 1e and 2. Those things aren't particular to slaves.
It isn't dehumanizing to consider someone "a source of information or expertise". It's humanizing in that those are predominantly human things. And that's one of the primary senses of the word. It seems uncharitable to assume someone must mean some other sense.
The meaning of a word is much more than its definition in a dictionary. Context matters. Humans are animals, yet saying that someone is an animal might mean anything from praise (You're killing it, you animal!) to contempt (Stop behaving like an animal!).
