There are two other issues with this...

1. The authors only studied two military camps. One with high TCE levels, and one with low TCE levels. But, obviously, there might be any number of other factors that are different between the two camps with a causal relationship to Parkinson's Disease.

2. There is an active class action lawsuit from this military camp. Doing a limited study that benefits one side of a class action lawsuit should always be taken with some extra skepticism and evaluated for conflicts of interest.