As I understand it, Lt Grayson was in a Marine counterintelligence unit (2nd CI HUMINT Co) and not under LtCol Chessani's direct command. I'm sure you know more about the relationship between attached specialized companies and regiments than I do.
I wouldn’t not be surprised that the Marine’s have a different system than does the Army. If an S-2 were assigned to a fighting unit from the intel battalion, in the Army they would still be rated and senior rated in the working battalion. The “intel group” assigning them would be in intermediate rater capacity.
In fact, that’s exactly how other special staff works in the Army.