Not disputing that adults should also remove bulky winter wear, but adult seat belts have pre-tensioners in newer vehicles. This tightens the belt when necessary in an accident. It's not the same as sitting in a static 5 point harness. I most definitely agree that it's important to have the lap belt in the correct position and not have a coat in the way. For adult with pre-tensioners I think it's less important to remove all upper outerwear as long as the shoulder belt is in the correct position.
If I live in a climate where it was reasonable to remove all outerwear in the vehicle, I would do so, but I don't live in those fantasy places.
You often need to wear your coat here for at least a half hour until the vehicle is warm enough inside (even if you've warmed it up for 10-15 minutes prior) and even if you take it off at some point, you've got to get out of the vehicle and walk to your back cargo area or trunk (unless you've place your coat on your 2nd row passenger's heads) in -22°F and fumble with the zippers while your flesh literally freezes due to the windchill (and no I am not exaggerating). Ya, not so practical here a good chunk of the winter season.