Sidewalks are a lot more dangerous for bikes than you might think. The problem is cars have expectations on how fast people approach and will often assume an area is clear based on walking speed. Further even low speed collisions with pedestrians can easily send a rider into traffic.
One might err in assumptions about what I think. Parent had "seldom used sidewalk that runs for miles next to a bicycle hostile road". Obviously that is different from a regular grid of streets, where no one should cycle on the sidewalk. As soon as crosswalks appear get your ass in the street. Even in the situation the parent described, however, the presence of one pedestrian makes it a sidewalk again so no cyclist will ride and no low speed collisions will occur.
