. . .Assuming Sybok is a Vulcan of some intelligence, why wouldn't he notice that Chekov ISN'T WEARING a captain's rank insignia?
Sybok isn't in Starfleet, he's a civilian. I'm a civilian too, and I would never be able to distinguish American naval rank insignia, but I would be able to for army and perhaps air force, at least for commissioned officers. I'd have less luck with rank insignia of other countries.