The author acknowledges that artificial fertilizer-based farming is “more damaging” than organic farming. There’s no indication the author thinks this damage is only slight as opposed to significant.

This possibility, if true, shows that it’s possible organic farming can spread without it extending to “all farmers.” And if it doesn’t extend to “all farmers,” then we have no reason to think we’d be in a position where it’s impossible to feed the world.

The premise establishes that in order to feed the world, we can’t have “all farmers” doing organic farming. What farming was like in the past and how much food such farming allowed has no bearing on what the premise says is currently required.

This isn’t a possibility overlooked by the author; it’s closer to something the author assumes. The author thinks that we’ll face inability to feed people if organic farming spreads even a little bit more. Since this possibility doesn’t hurt the argument, it’s not the flaw.

The author’s argument doesn’t assume anything about damage to human health. The issue is whether a further spread of organic farming would lead to inability to produce enough food. How fertilizers affect health doesn’t bear on this issue.