I believe the issue GP brings up is that merely inhaling a few breaths of this stuff is enough to cause damage. The crew may not have known beforehand that there was a leak or an issue.
I've sometimes gotten a whiff of a toxic fume, and my instant reaction is to hold my breath and leave the area. Not inhale a few breaths.
I'm sure the astronauts are fit enough they could hold their breath long enough to get the helmet on. After all, I expect they probably have a lot of training in getting the helmet on fast. There are many emergencies that would require this, such as rapid decompression, fire, smoke, etc.
I don't know if the astronauts do this or not, but if I was reentering, I'd put the helmet on anyway. It's only for a few minutes.
So no, you do not get to generalize here.