If you’re not interested in the Bible’s answer to the question who told the first lie then I don’t know what we’re talking about. Even if you think it’s fiction, I don’t know what the point is of imposing a different answer than the one given by the Bible itself. You could argue Dorothy in the Wizard of Oz wasn’t from Kansas because she didn’t have a Kansas accent. Okay but what’s your point. In the film she’s from Kansas.
If you’re making an argument about the original intention of the author of Genesis against what later Bible writers interpreted that story to mean, then I don’t think it’s as straightforward as you might imagine for a number of reasons. Who the original author was, what form the story originally took, and what it meant to the first audience is perhaps beyond recovery at this point. It’s entirely possible the original author meant something completely different than the text can currently divulge because it has been adapted and now appears in a context which it didn’t originally have. So it’s not as easy as you might think to say that later Bible writers got the original meaning wrong but you have got it right, and this is what it is. Even if you could be certain about original meaning of the Genesis story (presumably before there was a book of Genesis, an earlier source that we don’t have access to) by the author in its original form, whatever that was, it’s still different from the meaning it currently exhibits as part of the Bible as a whole.
While it’s true the Bible arose as a diverse set of documents that accumulated and adapted over time, it now appears as an edited collection and it makes sense to read it in that way. It’s not a coincidence, for example, that the book of Revelation recapitulates and resolves issues set up in Genesis, including the identity and fate of Satan. If you say you are not interested in the answer the Bible itself supplies to the question of what was going on in Genesis, who was the first liar, and so on, fair enough, but I don’t know what we’re doing then. The Bible itself says Satan was the original serpent and the first liar.