In car-choked cities the average car speed taking into account congestion may be lower than walking or cycling. So less of them will increase mobility for sure, both for those in cars and those using other tranportation means.
Good point. For me the issue is the variance which governs the time that I have to leave. For the subway, if I give myself a 10 minute margin beyond the median time, I'm quite likely to be on time, for walking or biking if I give myself a 5 minute margin, I'll almost certainly be on time. For driving on the other hand... as per the article, a trip with a median travel time of 17 minutes can take me over an hour when traffic backs up.