What you are missing is that having hundreds of Model X based mini-busses going through this tunnel is less efficient (both in costs and energy) than having one

What you are missing is that having hundreds of Model X based mini-busses going through this tunnel is less efficient (both in costs and energy) than having one subway train carry the same number of people. Now, if you want to be charitable, you could say that the Boring Company will bore a huge number of tunnels all across LA and install a huge number of stations. That would enable people to enjoy the advantages of individual transport (i.e. being able to go point-to-point) while still removing traffic from the surface streets. But that's not what is happening here. So far they are just doing a super inefficient mini-subway.