Actually, the plaintiff did not have to prove that he contracted cancer from exposure to the chemical, only that it was a likely contributor. Same is true with asbestos. You cannot prove beyond all doubt that exposure to asbestos caused a specific case of mesothelioma, but you can convince a jury that there was a reasonable risk of exposure, that the company producing it knew of the risk, and that it should have warned people of that risk.
I don't really understand your argument here. On what grounds would a case like this be barred? Who would make that decision? You've put the horse before the cart in saying that "there's no evidence that his cancer is due to Roundup". How could that be determined before such a trial and by what authority? If it can be determined without a trial, then why bother having one? Just tell us how it can be done.
With our current system, we do have an "expert" that makes that decision, the judge. The judge is supposed to be the expert on law. Judges can and do dismiss cases if they deem them to be frivolous (no evidence of wrong doing) law suits.
The judge is no "expert" on the alleged dangers of glyphosate, and this complaint was hardly frivolous, given the controversy surrounding conflicting claims over the harmfulness of the chemical.
However, it appears that the bar for deciding that a suit is frivolous is awfully high.
Not in this case.