I know a few people who are disabled, and several elderly people. They would not be able to judge a speeding vehicle properly as they are more used to a certain threshold of vehicular travel. This driver was way above a simple case of speeding and as such it is entirely plausible that the victim looked, saw the HGV in the distance and thought they had time to cross.
Another possibility has entered my mind today in that many times I have seen pedestrian standing in the road but not crossing, mearly waiting for a safe opportunity to do so). I wonder if there is a chance the guy might have done this, the driver panicked and the force of the vehicle overturning swept up the guy? Could this be plausible?