It's easier to understand if you think about the tunnels as lanes. Each tunnel can still have busses and cars, but you can also stack them on top of each other. You can do that above ground too, but it looks super ugly and you can't really go 10 stories high. Underground you can.

He probably shouldn't have driven a Tesla through it, the only message people got was that the 1% are going to have tunnels. Really you'd run autonomous buses and 4 person taxis through it. Easy to imagine it being much cheaper than public transit too.