I remember Elon initially saying tunnels were supposed to transport passengers in a Tesla vehicle, which could carry 8-16 people, and vehicle would be on rails/skates. Then it turned into just a Model X on rails and then it ended up being just a regular Tesla driving on autopilot
When Elon had the first demo of the tunnel it was bumpy and had the press fearing for their lives.
To me it looks like most of the cost savings is due to tunnels being smaller and not some breakthrough digging technique/machining.