Never really thought about it, but it seems kinda "duh" now. Why else would the arboretum in Golden Gate park, surrounded by city on three sides and ocean on one, be loaded with squirrels? They don't just fly in on their yearly migration.
It's funny that the article mentions that seeing a squirrel in the city was once a sure sign that it was a pet. Nobody thinks of squirrels as pets now. My uncle did have a pet squirrel for a bit in the mid 90s though: Ricky. He rescued him from falling out of a tree and raised him in his house. There would be a squirrel running around the kitchen. Eventually he gradually re-entered the wild: he would go out in the backyard and spend more and more time in the trees and less time in the house. Seemed to work out pretty well.