The helmet splitting is a result of the crush on impact. This is why you should always replace the helmet after it has been "useful" i.e. suffered an impact. Even if it stays in one piece, it has been crushed and lost some of its protection.

I think that the parent comment's statement "situations where a helmet has saved a life are rarely recorded" is a bit of an exaggeration, because quite often, if the helmet saves your head, you still suffer some other injuries. But it is indeed possible that people fall so that they suffer no other injuries, they'd just have cracked their head without the helmet.