You can see in that very video that it's not even an unsolved problem, it's an unaddressed problem. They currently handwave it away as "vent the molten rock" as

You can see in that very video that it's not even an unsolved problem, it's an unaddressed problem. They currently handwave it away as "vent the molten rock" as if that is a solved problem. At 6:34 in the video, they very briefly show a running test drill, and then cut immediately to the ceiling of the test chamber with a large specimen of rock wool. That rock wool will completely clog any mechanism they could come up with. How do you reliably transport rock wool from 20 miles underground to the surface? This project/company is a dead end unless they could magic away that problem.