Helmets are super rare in the Netherlands or Denmark, and they have much lower accident rates than the US, while having much higher use of bikes.

Lower than the US doesn't mean they are doing something right. The Netherlands still have far too many head injuries, and there have been groups of doctors pleading for helmet use, since these injuries can be prevented or mitigated. For example.

It's also important to note that the Netherlands have had an increase in cycling injuries over the last five years, mostly because of e-bikes. They really should be wearing helmets, no matter how perfect they believe their cycling infrastructure is - many of these crashes don't involve a motor vehicle (60%+).

People getting in accidents is much more about the lack of infrastructure.

And lack of critical mass. Biking in my college town always felt pretty safe... except on days that brought lots of outsiders to town. Those people don't know how to share the road with non-cars. Experience of regularly driving around areas with bikes means you get used to thinking about their existence and don't do things like suddenly turn right directly without even turning on a turning signal in front of a cyclist at the bottom of a hill.

These are all very valid points, but they don't explain why there is a noticable difference in behaviour to cyclists, despite their traffic conditions being the exact same (perhaps even more dangerous for scooters). The road being dangerous is also no excuse to become a danger to others by riding on the sidewalk. Of course, it would be best if the traffic conditions were more safe for micromobility. But as long as that's not the case, the reasonable thing is to protect yourself.