It's fallacious if you use it merely as "Authority X said so," no matter whether the authority is true or false. A fallacy happens when the conclusions do not follow from the premises of an argument. E.g. it doesn't follow from the fact that "Al Gore says that Global Warming is happening," that Global Warming is happening.

If, on the other hand, you say "Al Gore said Global Warming is happening, Al Gore's argument is premise A and B, therefore C" that's not fallacious because your argument doesn't actually rest on the identity of the authority, but the argument they made.

But all of this is irrelevant because it's not an appeal to authority. It's an ad hominem. The author is being attacked for not-being-an-authority.