The discouragement of biking is a big deal in the Netherlands. They’d rather have everyone biking casually than a few dedicated speedsters decked out with the right equipment.
I assume he didn't - the point is asking people to wear helmets discourages biking. In practice it's overall healthier to not ask people to wear helmets.
Having visited Amsterdam once, I'm honestly a little surprised there are so few cycling accidents. The number of bikes is huge and the flow of it looked a bit like a controlled chaos.
The Dutch seem to have understood the street smarts and the unwritten rules of cycling (as applies to their environment) really well, so that probably explains it, along with a cycling-friendly environment.