Indeed bicycle helmet use is very rare. There’s no helmet rental scheme, although in some railway station bike shops you might be able to buy one. People, I assume, will bring their own, as they will usually also cycle from their home to the railstation with their helmet on.
Afaik it’s just the two-wheeled variety, unlike the Hoge Veluwe you mention where they have a fleet of bikes for different use cases. They for a number of years experimented with electric scooters en e-bicycles, but that’s been discontinued after 2014. I think because of higher maintenance costs, and they probably were a bit early with those experiments. Might be different now that e-bikes and e-scooters are much more common in general than a few years ago.