How do you know that? More to the point, how do I know this happened as you claim it did :P
I went to A&E a few years back with a head-injury - 12 stitches in my forehead. I would GUESS a helmet would have mitigated that, so I am VERY aware they can help sometimes; but that was the 1st time in many decades of riding, and the reality is, some stitches are not the end of the world. Nature makes our heads rather well; that's why head injury clinics were not overwhelmed with cyclists back in The Dark Days pre-90s when no-one wore lids for leisure, and the modal share of cycling was much much higher. Pro Footballers play a few days after those oh-so-dangerous sounding "skull fractures".
Over to you, hit me with your risk assessment "facts" !