Re read the situation.
Very cold conditions (-20C)
.....and a sharp knock on something coul deasily create that with the material becoming more brittle at such low temperatures.
The trouble is, we photographers keep demanding lower weight, lower prices and higher performance. Something has to give somewhere. The lighter, magnesium alloys are not as robust as aluminium alloy or even brass or steel alloys which used to be used, but weighed a great deal more. The plastics that quite a few lenses are made from are less stable again, but in "normal" conditions will be fine. I would be careful if you go to the frozen wastes though.