Yes, you could probably share cars to a greater extent than we do today, but not nearly enough to get the ~20x reduction in the number of cars on the road that the parent suggested.

If you have optimized traffic, you could live further out and the commute wouldn't take any longer, so that could lead to more sprawl. You also would be able to do something while commuting besides watch the road, making the commute more tolerable, which could lead to even more sprawl.