[brwnll]

It doesn't appear anyone read the Author's linked article about the compression of the helmet and which forces it is designed to prevent. So here is a short snippet :

"If the styrofoam does not compress, it cannot reduce linear acceleration of the brain. The most protection that it can give to the wearer is to prevent focal damage of the skull and prevent minor wounds to the scalp. It is not likely to prevent serious brain injury.

This helmet has split along the ventilation slots, which is common. However, the thickness of the styrofoam has not been compressed. It most likely gave no more than superficial protection.

Some dissipation of impact force might occur from the action of a helmet breaking, but in most cases this is likely to be small. Helmet standards require the foam to start to compress at a level of force less than that which might be expected to lead to brain injury. While it is known that many helmets do not actually meet the standards to which they are supposed to be accredited (BHRF, 1081), it follows that if the styrofoam does not compress at all, the direct linear force on the helmet was minimal and it's quite possible that the cyclist would not have received any injury if the helmet had not been worn.