I think it means that the earth’s crust is a bit thinner there, so it is easier to tap into the geothermal heat below the crust. The deeper you go, the hotter it gets.
no vulcanoes aren't a location with thinner crust, but a place where a bubble of magma is stuck in the crust, because of a hotspot or because of subduction or other reasons. When the pressure is too high, or there are forces that push the magma upwards, it "leaks" some magma, some times violently, some times no, like in Hawaii. An inactive volcano probably has a bubble without enough pressure to release it, and that has no upward forces nor a way of building more pressure.
For those interested: since there's been a huge gas exctraction scandal in Groningen, (gas) mining has become a sensitive political subject in the Netherlands.
Also the Zuidwal volcano they are talking about is 2 km deep.
Geothermal energy is an interesting additional "green" option, if the mining risk can be minimised.