What I am getting at is the non-discovery of the GR by a society cannot be a sound argument agains the GR being a valid moral principle.
No one has said it is not. But it is not an absolute. In addition, the Golden Rule itself is a relativistic rule.
It would be necessary on your part to demonstrate the existence of non-GR societies of similar complexity to GR societies (that is, societies with complex state forms of government, relatively complex economies, at least Iron Age technology), function as well as GR societies.
Actually, since the Golden Rule supposedly predates the Iron Age, you are being overly restrictive. In addition, if you look at the history of what Golden Rule societies have done whenthey do come in contact with non-Golden Rule societies, you have to question is the Golden Rule itself is a moral guide.
Aslan is not a Tame Lion