I had never heard of this before, sen from afar it does look like some organic growth pushing up through the ground.
Those crowds though, yikes! I think I am not so surprised because when I was in Ibiza ,the old town and castle there which gets very crammed with tourists has residents living in it which just seemed nuts. Maybe as you say they go away in peak times. Or perhaps only venture out in the evening!
Yeah in this case it's a very small space with only a few streets so it doesn't take much to fill it up. In August in France a lot of businesses shut down for 3 weeks or so, so a lot of people go traveling, to the south or to their summer homes.