You should take a moment and measure the PH of an activated dose of Miracle-Mineral-Supplement . You will find that it has a PH of around 3. That is an acid.
When you add an acid to sodium chlorite, you form chlorous acid. Chlorous acid has some chlorine dioxide in it. If you want to add chlorine dioxide to water you need to remove it from the chlorous acid.
Water purification is done with chlorine dioxide and the PH of the water doesn't change. Chlorine dioxide is a gas that doesn't change the PH of the liquid it is dissolved into. On the other hand when chlorous acid is added to water the PH drops.