A few years ago a former colleague was almost killed when he was crushed by a car which couldn't see where he was driving. He was "lucky" in that he "only" lost both legs, he was standing at the rear of his van loading it up when the car hit him. He was some 6" 7', now in a wheelchair.
This is not a justification, but maybe an explanation: Perhaps they did clear the screen, but not the roof. Then when they braked it slid off. They should still have pulled over of course, and in fact they should have cleared the roof in the first place. Which is another lesson from this.
No show visible on other cars or the side of the road, and even if the screen was covered in snow when they started driving, surely it would have blown away at that speed?
Not saying that it's not incredibly reckless of course, but it does seem very odd!