The point is not to tell people to drive rather than walk. It's a point about the food system and the oil it consumes. That people who drive go farther isn't relevant to this analysis, because this is a per mile analysis to make a point, not an argument against walking. (Though in fact, I've often made the walk/bike vs. drive choice, especially when I worked 2 miles away and would bike commute half the time and drive half the time, depending on schedules and weather. But I made the choice for fitness reasons, not simply to be green.)

The point is how surprisingly inefficient we are as a system, using 400 gallons of fossil fuel every year to feed ourselves. (Because I don't commute, I probably burn about that every year to drive myself, but much more to fly.) What's amazing is that the comparison is even possible, not the fine points of it.