Cunningham will correct me if I have this wrong, but I don't think he's saying that Hit Girl does have actual superpowers. The point is moot - she is so ridiculously brilliant that she may as well have superpowers. It's so far beyond what training could ever accomplish for an adult, let alone an 11 year old girl - never mind that the only training we ever saw was taking a bullet in the chest. And this is a shame, because Cunningham is quite right - the film was working better when it was grounded is some kind of reality.
She was a fabulous character and an instant icon, but on reflection I agree it was her that undermined the film. I'd have preferred a conceit which relied more on being smart than having - effectively - superhuman skills. Only once did she really use her apparently innocent 11 year old girl persona, which was a waste. She could have so easily wrong-footed villains in different surprising ways - her age was her real fighting asset, but it was ignored at the expense of just going with the isn't-it-hilarious-that-an-11-year-old-acts-like-a-superhuman-killing-machine. Yawn.
One more thought - I didn't like the use of commercial music in the film. Compare with The Matrix, where the commercial tracks were so seamlessly integrated into the score and the film itself that you barely noticed them. The Banana Splits seemed like a really cheap shot to me, and served to take me out of the film - "oh right, ironic use of music, ha ha" rather then actually getting immersed in the story.
EDIT - cross post!