A time traveler from the 17th Century would be familiar with this sort of quackery. I guess not everyone can sell alchemy, so some make do with other branches of “science”.
From https://en.wikipedia.org/wiki/Chlorine_dioxide#Safety_issues... the EPA "has set a maximum level of 0.8 mg/L for chlorine dioxide in drinking water", which is certainly much higher than the amounts in MMS.
From https://en.wikipedia.org/wiki/Miracle_Mineral_Supplement (for example) - "Following a May 2010 advisory which indicated that MMS exceeds tolerable levels of sodium chlorite by a factor of 200".
Summarising, the idea of toxicity of chemicals and dosage was first explored by an alchemist (Paracelcus) in the 1500s, which is the 16th C. - so yes, 17th C. medics might have been able to point out how crazy this all is, let alone 21st C. people of any kind.
It's been 500 years, people.