No, it is not always a fallacy. There is a legitimate argument from authority.
Argument from authority - Wikipedia
Jesus is the revealer of Revelation and he says the serpent is the devil. He is God revealing these things, the Alpha and the Omega, the beginning and the end of all Authority.
Rev 12:9 And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.
Rev 20:2 And he laid hold on the dragon, that old serpent, which is the Devil, and Satan, and bound him a thousand years,
No, a metaphorical serpent would not serve the cause because a metaphorical devil would not serve the cause, just as metaphorical sin would not, and certainly an imaginary death would not.
I guess you have your rules for intelligence, so I guess I'm a dope. End of discussion.