I'd be very careful about assuming the legend was made up by Plato just because it's not widely attested in writings of the time.
It's likely that most of stories from oral tradition have never been put down to writing, and most of those who did were probably lost through the filter of time (along with the vast majority or ancient Greek literature).
It's likely that most of stories from oral tradition have never been put down to writing, and most of those who did were probably lost through the filter of time (along with the vast majority or ancient Greek literature).