You've probably already looked over 61.215 but it grants advanced ground instructors the authorization to provide "a reccomendation for a knowledge test required for the issuance of any certificate under this part"
So the question is, is the instrument rating a certificate? I'd venture a guess and say no, it's a "rating". Per 61.215, an IGI is authorized to provide "a recommendation for a knowledge test required for the issuance of an isnturment rating under this part." Therein lies the distinction, I suppose.