American post-war car-centric urban design transformed public space from one accessible to, and commonly ranged by, all (including children), into one largely accessible by motorized transit.
Children without licenses and access to vehicles consequently lost direct autonomous access to the core locations in their lives (home - school - simple shopping - peers - common wild space), and have been discouraged from attempting to navigate these spaces on foot (for fear of injury or unrealistic transit distance/detours required).
The author does not state, but one may infer, that car-centric living is coupled to the overscheduling and helicopter parenting at issue in the original article; and that children raised in a car-centric environment experience something akin to learned helplessness.
These problems are further compounded by comparatively poor American public transit systems and a contemporary mindset in which fearful parents discourages free-range children from making use of what transit options are present before the last few years of childhood; and by safety concerns around biking and skating in environments which do privilege cars.
As a parent these considerations are omnipresent in my household.
My own kids are in the top quintile of "most free range" in our cohort; that feels possible in San Francisco in some ways which do not appear as feasible in many other urban areas. That's one reason we continue to try to raise our kids here.
But they do not get anything like the benevolent neglect of my own latchkey neighborhood-wandering local-kid-pack-factions semi-rural/suburban childhood.