He probably means German tourists, often senior citizens with fancy gear and poor skills. For some reason cycling in foreign countries is in a blind spot, the Dutch travel a lot but tend not to notice the lack of cycling elsewhere and not notice the exceptional amount of cycling here when returning home. It has been sinking in for a while now but we need regular reminders from foreigners that we are the exception.

The Dutch aren't just the exception in helmet wearing, but in general. The people of Amsterdam are reminded daily that foreign tourists aren't competent cyclists, not only in understanding traffic but also in bike control, tourists are the main danger and disruption of Amsterdam traffic these days. Maybe the tourists should wear life vests too, with all those canals without fence and warnig signs. The Dutch have been on a bike from a very young age and almost every day of their lifes, they all have a lot of experience in taking care of their own safety and a lot of skill, especially when it comes to not falling and when falling (snow, ice, intoxication) not hitting their head. Upright bikes probably help, but I guess it's mostly about reflexes from a very young age.

Besides that helmet wear would disrupt the way we use bikes here, there's also a problem with others wearing helmets. You've got to rely on your own skills, but you've got to rely on other people's skills too. If you are not skilled or careful enough to ride safely without any protection, how can I rely on your skills or care to share the road safely with you? Or have my kids share the road with you? It's not just about personal choice.