What are they going to do with thousands of human prisoners of war? Unless they slaughter all of them, it will bind a good portion of the too few inhuman soldiers they have got. I suppose they could try to exchange them for the inhuman slaves, but let's be realistic: it wouldn't serve the purposes of that human government to agree to it. It's just a private army, so the government likely considers them expendable. Furthermore, as long as those thousands of prisoners exist, it will require a lot of guards, like I said before, leaving less inhuman for fighting. Should the inhuman kill them, it will be good material for further propaganda. In practice I'd imagine the human government would agree to a prisoner exchange, but they wouldn't actually do anything, leaving the inhuman waiting in vain, while the humans actually prepare for war.