As an RF engineer, this is a very creative application of mmW and I hope can be successful, but is not without very significant challenges which the company has made no claims to have solved. The issue is, mm-waves (and all high frequency RF really) are very sensitive to distance and losses. As a random example [1] gives some numbers for theoretical and precision machined waveguides in these frequencies. In the very best case we're talking about 0.03dB/cm loss. That means in the first meter of ideal waveguide you've lost half the power! And not just disappeared, you just dumped 0.5MW into the pipe itself, meaning it would likely melt within minutes. Meanwhile, their concept is to bore 20km into the earth.

The only way I could imagine this working is to lower the gyrotron itself down the borehole, but there's presumably tradeoffs of physical dimensions and output power that make it challenging. Again maybe not impossible, but there's no indication they've made fundamental advances in miniaturizing the source, or producing lossless waveguide.

Related, another interesting company I came across is Petra [2], who is doing something similar -- plasma/heat based non-contact drilling. But because they fracture the rock instead of vaporizing it, there's essentially gravel/sand leftover which is hard to evacuate from a deep vertical shaft, hence their focus on power/fiber trenches not geothermal.

[1] https://www.nrao.edu/meetings/isstt/papers/2012/2012151153.p...

[2] https://petra.cc/