Comparing single factors between Netherlands and any other country is fraught. They have a hundred years on the rest of us, with an endemic cycling culture and physical safety considerations to keep most cyclists separated from traffic.

The numbers of commuters on a bike involves more than requiring a helmet or not.

Slipping on ice isn't comparable. This is a low speed accident where you often get to control your descent and protect the valuables. In the UK, 85% of on-road cyclist fatalities involved another vehicle, and that remaining 15% will include pedestrian collisions; the vast majority of deaths aren't from simple misadventure. They involve some speed.

So why bother? Because being "designed for" falling doesn't means they only work for falls. The article was pretty clear here, they lower impact damage, turning fatalities into hospitalisations and hospitalisations into [largely] unreported bumps.

Correlating sports cyclist helmet usage won't tell you much. Sports cyclists are usually required to wear a helmet by their events' and their personal insurers. You train in the same conditions.

These are hard things to discuss intelligently. Even looking at a single country, it's nearly impossible to subtract other factors to see if helmet laws worked. But it's still clear that hitting your head is a stupidly simple way to die, and those same impacts don't kill you if you're wearing a helmet.