Having known someone who ended up with brain damage while riding at fairly low speed without a helmet (because of a dog and NOT traffic), these studies don't mean much to me. Just because a traumatic event is a rare one doesn't mean we shouldn't take precautions against them.

The safest biking cities are places like Amsterdam and Copenhagen, where middle-aged commuters are mainstay riders and the fraction of adults in helmets is minuscule.

The implication that those cities are the safest because of low helmet use or that helmet use has anything but a minuscule correlation to safety is ridiculous. Those cities are primarily safe because of culture and infrastructure.