The first issue we have to consider is the manner in which an impact can be absorbed by a helmet which is above the "test line", an inch or two above the lowest portion of the helmet. If the impact occurs below this test line, the helmet can't provide the protection it was designed to.
Does the impact occur within this range? Most often they don't.
The next point is, does an impact from a simple fall result in a crushed skull, resulting in death? Well, I'm not going to say it never happens, but I will say it happens to people off a bike just as much, if not more, which is to say, very rarely.
It is collisions with motor vehicle that kills people on bikes and the few times when it doesn't, high speeds (speeds beyond the limits of a helmet) and impacts with fixed objects are involved.
I'm always amazed at how many people stick with a theory rather than look at a reality and adjust their idea.
Australia and New Zealand have had close examinations that show when everybody uses helmets, injuries don't drop. Everybody can have their own thoughts and ideas about what they think would happen if everybody (or anybody) wore helmets but the reality is staring them in the face. Helmets haven't made a difference.
In my province, the legislators proclaimed long and loud about how the evidence (from a small case control study) was overwhelming that wearing bicycle helmets will dramatically reduce serious injury and death. So what happened when everybody did start wearing helmets? Cyclists continued to suffer serious injury and death, only now they were wearing helmets when they did.