I'm afraid I have some rather conclusive evidence to prove you wrong Brian. Dry ice works miracles. And here's the scientific evidence.
Following your testing parameters I too used a ball peen hammer to make a few rather large dents. I didn't happen to have a Prius fender to duplicate your experiment in exact detail, but I was able to locate my dog Buster's forehead to use as a comparable substitute. The results are shown in picture # 1.
I then bought $0.59 worth of dry ice (exactly as you did) and gently applied it to the dent for about 30 seconds. And here are the results.
Now I just need to do a little touching up with some Rustoleum and foam roller and he'll be as good a show dog as he ever was.
(Helpful Hint: I think the secret for using dry ice is to rub WITH the hair, not AGAINST the hair.)