From the thesis: https://www.proquest.com/openview/624989df3cdd8055a6cee9affc...

"For the application in EGS drilling, this device uses a metallic waveguide to carry the millimeter wave (MMW) beam to a standoff distance from the crystalline rock. Argon gas is used as the waveguide fill medium due to its ability to stay transparent to MMW’s at such deep depths and thus higher pressures [12]. Purge gas is also used to pump out the excess material that has been transformed into smaller particles (Figure 2.4). "

As a former geologist involved in drilling, thats going to get real expensive, real fast, in terms relative to regular mechanical drilling thanks to the requirement for argon. Perhaps theres an economically efficient changeover point at depth as mechanical drilling becomes less capable due to increasingly plastic deformation.