Being hot is a disposition to burn. But, being hot consists in molecular movement.
Molecular movement occurs at a certain speed.
One would not, however, conclude that molecular movement is not a disposition to burn, on the grounds that a disposition to burn is not something that occurs at a certain speed. The part of Robbie's position that I don't follow is his jump from beliefs having to be true or false to the conclusion the believer must be able to know they can be true or false. I don't find the premise objectionable. Thus, I would agree that if moral relativism is true, then statements about what is moral are incoherent.