The whole point is that microwave ablation of these materials does not require drilling mud and is self-stabilizing by the pressures and molten material generated, while also sealing the bore hole and preventing quite a lot of water table, H2S, etc contamination. In theory it should allow substantially deeper boreholes.
The technology is real, and the required power density is surprisingly low.
They'll be pumping neutral gas (probably N2 or argon) to purge the borehole. I would expect the ablated material to resolidify as fine particulate, which would get carried to the surface at ambient temperature and trapped in a filter.
A 20km borehole with a 10cm diameter is only ~157m3 of rock. If the dig takes four weeks (which would be unprecedentedly fast), you only need to purge ~64cm3/s, which is pretty trivial.
They'll need to do something like mix the dust into concrete. Silicate dust is a respiratory hazard. But that doesn't strike me as a significant hurdle.
The technology is real, and the required power density is surprisingly low.
https://www.researchgate.net/publication/286571247_Penetrati...