Granting the premise that there are no absolutely neutral parties, we can still agree that there are parties which are less neutral than others. Having a direct financial interest is a pretty hard line: for example, if the author was heavily invested in geoengineering companies, or had even been paid by geoengineering companies to write the article, I hope everyone would agree that his opinion was suspect. In this case, the author does not have a direct financial interest, but he has (I suppose) an indirect one, which is why I say I appreciate the article, but would prefer the same argument from coming another source, if I had the choice.