Edit: To be clear, I was riding years without a helmet until a crash made me reconsider my priorities. Also, I am not in favor of mandating helmet usage on bikes.
All kinds of race car drivers were helmets - and their steel cages are much much more sturdy than the ones that commuters have. Statistics als indicate that many car drivers suffer head injuries. So by all available evidence, wearing a helmet in a car is not redundant. Still, people don’t do it - unless they engage in particularly dangerous activities.
The parent poster that you refer to explicitly gave his speed as about 20km/h for commuting, which is neither particularly fast nor particularly dangerous on good infrastructure. So why wear a helmet?
For me this discussion is done. I have this particular perception, you have yours. We are both happy with it. Stay safe.
I understand fear of someone who actually was in one, but please stop forcing your fear on everybody else. Just because you are afraid does not mean not being afraid is irresponsible or any more irresponsible then not having helmet in car crash.