It's substantiated by the fact that humans don't have radar and ultrasonics, just two cameras on a swivel and a lot of signal processing, and they succeed at operating a car to five-nines reliability measured in miles traversed. So Musk's bet isn't completely bonkers; he knows of at least one reference system that does the task with fewer sensors than even shipped with the Tesla.

... but we do want the SDC to do better, and there are failure modes that human perception is also vulnerable to generally. In addition to closing the gap faster on solving the problem without a copy of the human perception wetware, the LIDAR signal might also improve on those perception error states and be worth keeping in the design even if it could be done with cameras alone (or camera + radar + ultrasonic).