I do agree that it's probably nowhere near the top of the list of issues preventing this thing from working at least as well as conventional drilling technology.

However, anything about radar, ultrasound, or laser ToF would require electronics at the head of this waveguide and a way to communicate data to the surface. From what they're saying, the downhole environment of this thing is going to be very high temperature. Physically vaporizing 100% of the rock to make hole tends to do that. Conventional oilfield electronic tools already have trouble getting the MTBF above a few hundred hours at current downhole temperatures, which are much cooler. It seems likely that no electronics would survive at all at the temperatures they're planning on running.