Skepticism is healthy. You've found that the numbers don't make sense at face value. The problem is that you stopped there, you haven't even made any attempt at reconciling them with the original claim.

What if the US number of 1 in 400 figure is that high precisely because it includes people exposed to pesticide? In other words, maybe the number would be 1 in 500 if it weren't for Paraquat? You'd have to look at concentration maps or at the very least check what's the diagnosis rates in other countries before you can truly dismiss the claim, imho.