I'm arguing that there is a sense in which one "should" follow rules which has nothing to do with human-like agents laying down the law, thereby refuting NMJ's attempt at a link between objective morality and theism.

There are constraints on the rules games can have (eg fairness, a clear winner after finite time). There are constraints on rationality (eg avoidance of quodlibet). Likewise, there are constraints on the rules of moral reasoning. (eg. people cannot just make up their own morality and do what they want). Note that I am talking about metaethics here.