Another poster commented on Chekov's moments as acting captain of Enterprise in TFF and the dialogue with Sybok. Assuming Sybok is a Vulcan of some intelligence, why wouldn't he notice that Chekov ISN'T WEARING a captain's rank insignia?
Even if Sybok did
notice, it doesn't mean anything. Captain is both a title and a rank, and Commander Chekov would be allowed to identify himself as Captain Chekov when he's in command of the Enterprise