It's ad hominem; the author is being attacked to invalidate the argument, instead of attacking the argument itself. Which is never valid. Appeal to authority u

It's ad hominem; the author is being attacked to invalidate the argument, instead of attacking the argument itself. Which is never valid. Appeal to authority usually takes the form of skipping making an actual argument for something and instead resting upon "X said so" and not explaining further. This is also not valid but is sometimes a shorthand for saying "X made argument Y," which can be valid if argument Y is valid (in which case the fact that X made the argument is purely incidental trivia).