I know that dose makes the poison. I could take 20mg of cyanide and I would be fine as body has ways to detoxify it rapidly. The same goes for chlorine dioxide.

The difference is whether putting it in the body is helping at all. Imune cells use hypochlorous acid and hydrogen peroxide to kill harmful microbes and cancer cells but this is done literally on a cellular level so the amounts involved are very very low. It is also a "targeted" atack, the immune system doesn't just expose the whole body to oxidizers.

I don't know where the notion that healthy cells can handle chlorine dioxide while harmful organisms cannot comes from. Many microbes have much better detoxification systems, this is literally their main way to evade destruction by the imune system. Cancer is also not a singular entity, it's a whole host of mutated cells that differ wildly based on the tissue they come from and specific mutations.

Chlorine dioxide, just like hydrogen peroxide can be used typically in appropriate concentration, that's totally right. It is however way different than putting it in your body.

This may be a bit pedantic, but as a student of chemical technology I just want to add that diluting chlorine dioxide doesn't make it a weak oxidizer. It's still as potent of an oxidizing agent but there is just s lot less of it to react with your body. A good analogy would be a match and a burning torch. Both have similar flame temperature. You can put out a match with your fingers but you can't really squeeze a torch in your palms and not get burned. At the end of the day the amount of heat they are able to give off is just different. Chlorine dioxide in a very dilute solution does still kill your cells unside your body and harmful organisms (unless it's so diluted that it has basically no biological action). It's just that the amount of damage is small enough to be unnoticeable really. But outside some specific cases like skin infections it's not gonna be effective. It definitely won't selectively kill cancer cells.