I think the implication is that the rest of the damage is secondary. Yes, the helmet will protect you from the initial impact, but it also manages to take one for the team when you're sliding as well. And you will slide further, all things being equal, with a faster impact.
Years ago, through a comedy of errors, I was the pilot of a motorcycle and crashed while wearing a half helmet. My jaw came into contact with a truck as I went down. If I'd had a full helmet, I wouldn't have had a bruise from front to back on my jaw for the next week, and a bone chip missing out of my jaw today. I don't think the helmet ever touched anything.