Not intirely true.
Watching the old documentaries of the days of the depression, people raised roof top chickens and gardens on the roofs of abandoned buildings. They did a grand job of it, fed themselves and the neighborhood...
Small problem with your theory.
Back during The Depression, most people were barely a generation removed from the farm. Most KNEW how to butcher a cow, plant a crop, process and store their own food.
Today, how many urban residents have actually SEEN a cow? Most would be hard pressed to know which end of the cow to put the feed bucket under, and which one to use the shovel.