80%+ of severe injury and death on a bicycle is caused by motor vehicles, or complications of motor vehicle involvement.

Which would mean ~1 in 5 have absolutely nothing to do with a motor vehicle. That’s significant.

There is considerable evidence that everyone wearing a helmet in a car would save vastly more lives and prevent severe head injury

Then that should be an easy [citation needed] for you because my searches are coming up blank for actual studies. Lots of assertions of it, but I’m not finding anything in terms of actual data.

It’s very easy, on the other hand, to find comprehensive meta analyses on the efficacy of helmet use.

It’s also worth noting that the introduction makes a point of calling out another common online assertion that you repeated – that helmets make people engage in more risk-taking behavior – as false:

There has already been an extensive peer-reviewed literature review conducted by Esmaeilikia et al.5, which found little to no support for increased risk-taking when cyclists use helmets and if anything, they cycled with more caution.

I don’t feel those people should be called stupid for their choice.

I don’t think they’re stupid. I think they’re bad at risk analysis. That’s a pretty inherent feature of humans. It’s the reason I want to see actual data.