Sodium chloride does not remain as sodium chloride in the body. It is split in to sodium and a chloride atom where it is reformed in to other substances. For instance, some of the sodium is converted in to sodium bicarbonate, which is used as a pH buffer by the body. Some of the chloride combines with hydrogen to form our stomach acid.
Chlorine dioxide is quickly broken in the body forming chlorite. It then is converted to chloride ions. These chloride ions are reactive will find other things to bind too.
And yes we need chloride ions. But we also need vitamins, water, and oxygen all of which can be toxic in higher doses. Chloride is no different. High doses of chloride can cause anemia, weaken blood vessel walls, reduce our ability to fight cancers, can raise blood pressure, etc. The risk of toxicity increases in certain circumstances such as with congestive heart failure.