Not exactly true. Even the author of the article you linked states that gender may have a biological basis. The problem with relying on sociologists is that they look at the way that society treats people of different genders, and the expectations it has for their behavior. This is what sociologists do. It is not useful, however, in determining the underlying causes for the gender differences. They are likely biological in origin. Behaviors can be modified by expectations, but fundamental identity is innate. I think it is misleading, and perhaps dangerous, to try to discount gender as a merely sociological construct. This just feeds into the arguments of the transphobes that being trans is a choice or lifestyle, not a fixed, legitimate, internal identity.