I think the theory goes too far, tbh. If we're being a little bit realistic, a difference between animals and members of human society is that animals cannot and do not obey laws. Higher order animals obey direct threats of punishment, such as if you've trained them not to shit on the floor, but that's not the same as law. With a law, you are aware of the consequences without having directly experienced them.

An animal only respects consequences after directly experiencing them.

I am willing to accept into society those who obey (just) laws without directly experiencing the consequences. They ARE better than animals, they are not savages, they are not bad members of society. They are doing the bare minimum necessary to belong to society; indeed, their existence is the reason we form societies at all. You might not want to be friends with them but they aren't animals.

While the conclusion of this post goes to far, I do think it nails it right in the first sentence: if you return the cart, if you do what is correct without need for a law, then you are capable of being self-governing. You would make a good anarchist, for example, because your social group would function well without laws.