This is pretty much the definition of an ad hominem argument. In the context of your post, calling Taubes a "journalist" is overly reductive and an attempt to discredit him without actually engaging his arguments. I would liken it to dismissing Bill Gates' thoughts on education by calling him a college dropout - it's a true statement but ignores the larger truth. According to his bio, Taubes studied applied physics at Harvard and has an MS in aerospace engineering from Stanford so I think he's capable of understanding scientific issues.