Rezoning the area would be necessary, you are correct. The freeways which go through the city (instead of around it) are mostly elevated, so I don't think that's as much of a problem. And cutting most of those 4-lane streets down to 2-lane and using the reclaimed space as affordable housing or mixed-use fits in with the author's point.

This obviously isn't an overnight project, or even a ten-year project. It would probably take decades to rebuild a car-centric city for affordable housing and walk-ability. It would be easier to start from scratch, for sure. But existing large metro areas have a housing and congestion problem now, so need to start solving it somehow.