I think the point is that if you're a moral relativist across the range of morals encountered in human societies you're hardly a moral relativist at all because

I think the point is that if you're a moral relativist across the range of morals encountered in human societies you're hardly a moral relativist at all because from the space of all possible moral positions you've accepted as equally valid the tiny subset that have organically originated from the extreme restraints of human culture. It's far rarer to see moral relativists for example who think the moral positions of serial killers and humanitarians are equally valid, but even that range is small across the landscape of all possible moral positions.