In the presence of UV light, chlorine dioxide breaks down to chlorine and oxygen. This does not take place in closed loop systems.
The reason THM's are formed in the cooling systems of power plants is that the water is cooled in cooling towers that expose the chlorine dioxide in the water to UV light. This breaks down the chlorine dioxide to chlorine and oxygen, and the chlorine reaction forms THM's.
It is significant that the EPA limit for THM concentration is 80 PPB. You will notice that chlorine, at 1 PPM produces around 65 PPB, whereas chlorine dioxide, at the same 1 PPM only produces 0.8 PPB. In the Italian power plant, the design was to run 0.1 PPM, which would reduce the THM concentration to 0.08 PPB. They actually ran the plant a levels lower than that (0.05 PPM).
In Brazil the water had a higher microbial load, and they had to use a higher concentration of chlorine dioxide. Fortunately this was only for plant cooling and utility water. Drinking water standards would not allow this level because of the chlorite levels.
You will also note that in all of the studies the levels of THM's reported to cause problems were many times greater than the 0.8 PPB reported in the cooling water system. In a closed loop water treatment system, this level is further reduced, however the reduction, while near zero, is not completely eliminated.
Did you find the hepatitis article from the 1950's?