Comparing the Netherlands with some of the best, if not the best bike infrastructure to other countries without said infrastructure seems very reductionist. To get anywhere near an interesting number you would have to compare the number of injuries to the total number of accidents including cyclists in countries with comparable bike infrastructure and differing helmet policies.
Yeah that's why it's hard to prove. However, it does show that infrastructure experts who actually care about safety (and have achieved the safety numbers to support it) reach for a lot of other measures before they do helmet mandates.
