They're being a little oddly vague. It's all about the amounts of each that they kill. Bleach cures cancer, it just kills the patient too. And there's no such thing as "refining" something that used to kill healthy cells so that it doesn't ever anymore. Maybe they found a variation of the chemical that is strongly biased towards killing cancerous cells but unless that was their targeting mechanism the whole time, they're going to be killing some healthy cells too.