The cited value is the energy flow through the surface, which is about 0.1 W/m^2. But this is conducted through kilometers of rock and soil, which acts as insulation. Geothermal power works through convection instead of conduction. You inject cold water into a borehole, and hot water (steam) comes back, and spins a turbine.

Convection can extract energy at a much higher rate than conduction through the crust.

If you boil a pot of water, you can still comfortably hold the pot's handle if it's long enough, indefinitely. This is heat conduction. On the other hand, if you try drinking from the pot with a straw, you'll find it very painful. This is convection.