The big trick is to keep the drilling equipment and heat processing equipment from melting .... doubt that the hardest steel or alloys can stand million degree heat.
No need for million degree heat tolerance the earth core is under 10,000 degrees in whatever scale you use. Thats 6000+ km to the inner core no need to go anywhere near that deep. Depending on where you are on the planet 400C degree heat is “only” 3 to 10km deep a tiny fraction of that distance.
Here is the geothermal map for the USA all those red areas are 3km or less to power plant turbine grade heat as in 400C at those temps go supercritical turbines for water its 380C , CO2 is 300C either will pull 10 to 20 times the heat up a bore hole due to lack of phase change having the density of a fluid but the free flow of a gas. Using CO2 removes the leeching of water soluble minerals from the host rock and also has a lower pump energy to move a given volume of heat per fluid flow velocity.
https://www.americangeosciences.org/critical-issues/maps/geothermal-energy-resource-US
It would be a massive breakthrough to have 200+ feet per hour penetration rates in crystalline basement rocks that eat up diamond bits like candy. The earth is hot at depth due to the decay of radioactive material in the core and mantle rocks plus gravitational collapse energy from the formation of the planet. While there is only 4ppm on average of uranium in crust level rocks the core is believed to be loaded with uranium as it’s one of the densest naturally occurring elements nearly all of it in the original molten earth at formation would have density separated like the iron nickel of the inner core did at the same time. The earth produces neutrinos at a substantial rate indicating that either radioactive decay or active fission is occurring continuously in the inner part of the planet. Simply put there is no way humans could ever use this amount of energy its Billions of times what humans use in a given year. The authors are right 0.1% would supply all of humanity energy needs for tens of millions of years the amounts of energy is effectively limitless from a human time scale or usage amounts. Its like looking at the sun and saying humans can exhaust that source. The sun also supplies millions of times what all of humanity uses over the surface of the earth every year. So much so that ome hour of sunlight is enough to power humanity for more than a year. When you start talking planetary science level systems the amounts are beyond what most people can comprehend.
If you’re getting down far enough to where drilling equipment melts, it should be hot enough to harness steam to run turbines and generate electricity.