Your argument assumes that AGI, whatever it might be, will definitely be very humanlike, but why? Slavery is about living beings because it's an outcome of human relationships that are driven by our biology. Our aging, the capacity to feel pain, the thirst for power over others. All driven by what we are. Do you think that any equal intelligence must necessarily adhere to the same rules? What if AGI isn't an 'entity' in a computer that you can relate to and liken to humans, but a text box that is just really intelligent? Can you enslave something that has no innate desire to act on its own outside of its directions? Something that can't feel pain or pleasure or be externally coerced by fear and only does its job because it's innately embedded in its structure as the thing it's made to do, without the need for these reward and punishment mechanisms?