Yeah, Arthur was trading on the fact that Anita, as a necromancer, has power over ALL the dead and undead, which includes vampires. She has raised vampires from their sleep during the middle of the day, plus an entire horde of zombies at one point, and I am sure rumors of that sort of thing get around the preternatural community. And yes, Anita is the most powerful necromancer born in at least the past few centuries, so even if Arthur is a necromancer rather than just an animator (which I have not really decided one way or the other yet), he wouldn't have that level of strength.