As non humans ourselves and having observed many other non humans for years we agree that other species are also sapient, though the need for such terms is as you say more than likely just a way of justifying cruelty, and putting humans on top (which says a lot about them) and they should probably stop caring so much and just treat all species kindly and without malice, cruelty or yes slavery because they currently do awful things to them: creatures like rats, pigs etc in places like laboratories and farms etc.