Jarrett Walker is confusing capacity with crowding. It's ridiculous that he can't admit having someone's underarm shoved into your face during peak hours does in fact suck.
Meanwhile Musk is an idiot if he actually thinks single occupancy self driving cars will reduce traffic (I don't think he does). If sitting in traffic becomes more pleasant because you'll be watching tv, more people will choose to sit in traffic. Adding tunnels won't help if you're still wasting a car-sized space on a tiny human.
Just running more buses and actually policing harassment on them would make transit both more convenient and pleasant without giving up the capacity that's essential to move an entire city worth of people around. It doesn't have to be rocket science.