You are looking for what old-school grammarians might call the "pluperfect tense". That verb construction describes a state of being in the past in which an action was performed in the even more distant past and was completed.
I had walked.
He came to me twenty years ago. At that point in my career, I had walked many thousands of ...
Is Ben talking about the past? Or is he offering his grandma for tomorrow?
I deduce the following from your questions:
If Ben was talking about the past, then the sentence would accompany an earlier date, and would've changed to Oh. My grandma could swim back in the 80s [or whenever she could swim].
To answer your second question, I've noticed that there ...
No. That doesn't make sense. But you could say you tried to try the ice cream, but since it fell to the ground before you could get it into your mouth, you have not tried it. As far as the taste eluding you, well that does not negate the fact that you tried the ice cream.