Debunked urban legend. Likely retcon from various mythology involving the chemistry lab's "deionized water" bottle which every chemistry teacher has to make up convincing reasons for the class not to drink from.

It's mythological biochemistry. It does not reflect empirical evidence.

Exercising in hot weather requires drinking electrolyte water to avoid hyponatremia. Drinking large amounts of soft or distilled water (around 6 liters) can lead to death.