 On a second reading of Gibbard’s argument it emphasizes the distinctive practical or action-guiding nature of moral concepts. What any purely descriptive account of the dispute leaves out is that the parties disagree about what to do when in a prisoner’s dilemma. This version of Gibbard’s point aligns with Stevenson’s – there is a kind of practical disagreement that purely cognitive analyses of moral arguments leave out.  If that is the point of adverting to disagreement we are back with the motivating concerns discussed in section 3.3.  So it may be best to just think of disagreement as highlighting these prior ideas.