Even if you assume 4 people per car, cars still wouldn't work as well as the subway does for moving people. The original post is an interesting perspective on how important the subways are to NYC, nothing more.
Interesting mildly off-topic side-note (gathered from 'The World Without Us'): if the systems responsible for actively pumping water from the subway system were to shut down, the subways would be flooded in about 2-3 days (since they're well below the water table of the surrounding Hudson and East Rivers).