In the Basic Trainer's guide, under where it talks about the attack stat, it says the Dark type is Special, when only one attack is Special (Dark Void)

Because in the first 3 generations, that's how it works. Obviously, since the 4th generation reworked the formula to be based off the Physical/Special attribute of the attack, the part of the guide that refers to damage done based on the type of the attack doesn't apply. That doesn't mean it's wrong, it simply means you're misusing the information.