I guess the YVR 'authorities' are still a bit twitchy after the recent bad press.
I don't think so. Except as described, the roads and bridges aren't that close to the ends of the runways so overrun woudn't not be a consideration and I think the notion that they were closed because "a stricken airplane" might hit them is a bit extreme.
Vancouver airport is located on Sea Isand and is served only by bridges. What makes more sense is, the bridges and their access roads were closed to facilitate emergency vehicle traffic in case of an accident. Unlike city streets where there are places to pull over, if traffic is jammed on a bridge there is much less room to permit passage of fire and ambulance traffic. There are only four routes out of the airport, (two to Richmond, two to Vancouver). At the time of day that this happened, the bridges were a parking lot in rush-hour.