I agree with your thoughts. The page does not specifically call out for Parents and Grandparents.It is more like an indirect request. The CIC page where this information is available is for sponsoring spouse and not parents/grandparents. I wonder why CIC didn't create a separate page clarifying what country-specific documents are required. The email looks like a generic message that CPC sends out to all sponsorship applicants irrespective of spouse/parents/grandparents.
Did you receive a similar request? If so, what course of action did you take?