Bleach is a legitimate way of sanitizing drinking water. It's not a particularly long stretch to get from the sensible idea of properly using bleach to kill pathogens in your drinking water to the idiotic idea of using bleach to kill pathogens in your body. Even more so if you don't subscribe to germ theory, and bleach just "magically" improves the safety of your water supply.

Edit: disregard. Apparently, they were using an entirely different chemical. I can't even fathom how they could have thought it would work.