I can say for sure that the towers are indeed connected to the ground or counterpoise wires.
The reason is that I in the late-90s designed the 518 kHz NAVTEX antenna which uses the third tower from the transmitter as a 1/4 wave vertical radiator, creating
a folded monopole antenna.
The design and implementation was made in cooperation with Bengt Dagås, the station manager and chief engineer, and worked exactly as predicted.
In a small hut at the tower base, which originally housed a mid-50s FM broadcast transmitter, two Nautel 1 kW solid-state MFtransmitters are located.