You are correct. Chlorine dioxide is always a gas. However, it can be dissolved in water under certain conditions.
Chlorine dioxide, in higher concentrations, is explosive and very dangerous. It can not be transported. To get around this, sodium chlorite is formed. Sodium chlorite is considered stabilized chlorine dioxide. Sodium chlorite can be transported, and when it is activated it will produce chlorine dioxide. The amount of chlorine dioxide available from a sodium chlorite solution is related to its concentration. A 5% sodium chlorite solution has 50000 PPM of available chlorine dioxide. A 28% solution has 280000 PPM available, and so on.
When the PH of the solution is neutral or acidic, chlorine dioxide does well as a gas. When the PH becomes alkaline, it breaks down to the chlorite ion (ClO2 -). It is more effective as a disinfectant as a gas, but it is still superior to chlorine as the chlorite ion.
As you have discovered, the higher the concentration of chlorine dioxide, the more of a chance that it will have a reaction with organic material and form THM's. I believe this is why Jim Humble pays lip service to the 1 PPM concentration in his suggested protocol, yet he overlooks this when you actually measure the concentrations involved. The meat packing industry has demonstrated that higher concentrations of chlorine dioxide will bleach tissues, but since the carcasses were dead, there was no study of the effects of the THM's produced.
Keep in mind that Miracle-Mineral-Supplement does not technically involve chlorine dioxide technologies. It is actually acidified sodium chlorite technology. There is quite a bit of difference, and there are no formal human studies involving acidified sodium chlorite ingestion by humans. The best you can do is review the information on the various forums, but most of these reports have not been evaluated by a medical professional, so we really don't know what is going on.
Tissue samples taken from animals subjected to high concentrations of chlorine dioxide gas (not similar to Miracle-Mineral-Supplement ) have not mentioned any build up of THM in them. However, I am not sure they looked...
The reason I asked about the article is that in the 1950's, people would throw chlorine and chlorine dioxide in together, and they didn't understand the differences between them. There were many cases of people getting sick from impure water, however I am not aware of any of those cases where chlorine dioxide was used as the main disinfectant. These cases lead to better filtering and settling to remove organic material from the water so the chlorination would be more effective. I have only been following water purification for about the past 30 years or so, and I haven't looked back beyond that. I may have to do a little digging into the archives myself.