As stated earlier, there is no extant evidence before Plato.
Whether he heard a story and passed it on or made one up from whole cloth cannot be established.
People are certainly free to theorize on no evidence. In fact, it makes it much easier.
Man, if you’re looking for “strictly fiction” about ancient societies both real and imagined, there’s no better person to begin with than Chariots of the Gods author Erich von Daniken.
That dude is so out there! I mean, I’m all for disseminating new ideas to wage the war against ignorance, but this guy’s crock overflows with so much BS that it actually sets the rest of us back; it’s like cleaning your fingernails with a felt-tip marker!
In Chariots, von Daniken argues that all the megalithic cultures around the world that achieved massive public works (Pyramids of Giza, ziggurats in Central America, etc.) couldn’t possibly have done so without assistance from beings from another world.
His latest research, upon which he expounded at a lecture in nearby Plano, Texas (I couldn’t find my tin foil hat so I elected not to attend), reveals:
It’s sort of cross between The X-files and Clash of the Titans, I guess.