I actually think that the creation story is more than just a story. Luc seems to speak like it is true when he talks to the Flame Champ at the end of Suik 3. He says something along the lines of "when Sword and Shield fought in the beginning."
Luc is only referring to the myth when he makes that statement.
However, in this case, Luc seems to have pretty good inside information from the True Wind Rune. Luc obviously was very well in tune with his rune and was able to learn a great deal from the rune, and I think the beginning of time would be one of the things that he learned.
All the True Wind Rune showed Luc was a partial vision of a possible future, not the exact future. There was no mention of him learning about the beginning of time through the True Wind Rune. Besides, the True Wind Rune would have no knowledge of the beginning of time according to the myth because in essence it would have been created after the clash of the Sword and Shield. It would have no recognition of the supposed Chaos or the Darkness.