Actually it does say grandparent:
Option 2 – Other relative
You may sponsor one relative, related by blood or adoption, of any age if you meet all of the following conditions:
-you do not have a spouse, common-law partner, or conjugal partner, or one of the following living relatives you could sponsor instead:
son or daughter,
parent,
grandparent,
brother or sister,
uncle, aunt,
nephew or niece.
-you do not have any of the above-named relatives who is a:
Canadian citizen,
permanent resident, or
registered Indian under the Indian Act.
Since there is an existing immigration stream for grandparents, OP could sponsor them instead so wouldn't meet the "other relative" criteria. According to this, having a living grandparent here falls into exactly the same category as having a living parent.