All very well, but (a) helmet makes your head bigger (nearly twice as big) so more likely to hit it. and (b) helmets don't seem to work in Australia for some reason, so what are we to make of that?
So your saying if they weren't wearing a helmet chances are they wouldn't have suffered a head injury as their head would not have struck whatever it was it struck. - now a helmet is only an inch thick, so head-object more than inch away no contact, head -object upto an inch away hits helmet, so the object cannot get closer to the head than a fraction of an inch or it would hit the head and so the helmet thickness would not have been a factor. - so the object can only have hit the head within a 1 inch zone and not gone beyond that zone. which means the impact that head is going to take is pretty minimal as the head will stop getting closer to the object. - (its only got a 1inch travel distance before it would have hit the object without a helmet on)
so as the head would never have made contact with the nasty sharp object it will get a bit of a jar as it comes to an abrupt stop - and then what?
Now of course you can never predict what happens in an accident - that jar may be enough to cause a concussion , on the other hand by stopping the head before it reaches the end of its arch it may stop a nasty neck strain injury.
its all luck in the end, - as they say - if your numbers up , your numbers up ;