Whilst I agree with your comment is this not original research One has to assume Lucas knows about filmmaking and what the names are for things involved in filmmaking Its not a slip of the tongue  he says RD is a MacGuffin and then explains that in the industry that means something that everyone wants and so causes the characters to do certain things That it might not be the a MacGuffin sensu strictu per Hitchcock then perhaps sensu lato RD is a MacGuffin in the sense that at least some movie professionals mean it Cheers Neale