I think you have to try to put yourself in the mind of someone who is receptive to conspiracy theories when replying to people like above.

What you said makes sense, but it is only convincing if you already trust the scientific institutions and believe in reductionism. And that, is unfortunately much less common than we would think.

The guy believes that there are poisons in everyday products, telling them that "damage from an endless assortment of factors cause cells to enter disease states" is not at all a refutation of their belief taken from their point of view. In fact, I expect that it even looks like weasel wording for exactly what they said from their point of view.

I think that on topics like this, it's important to expose the premises, the concrete basic facts much more than the conclusion, the general abstraction.