Ok, I don't really like fan-made "theories". Sometimes they're right, sometimes they're not, but I just don't like speculating that much. I still want to comment on my opinion about the myth described in this chapter (I'm more than open to the high possibility of being wrong).
As for what's described in the myth:
First world: the part about "within the earth threre was fire. the mankind succumbed to greed and touched the forbidden sun" could refer to the mother flame; the "slaves prayed and the sun god appeared" part is more than obvious (nika/joyboy); and the part about the earth god and the fire serpent could refer to imu using an ancient weapon
Second world: this one is harder for me to understand, but the part about the forest god nurturing demons makes me think of the akuma no mi (I'm not sure about this part); the ones on the half moon could be the kozuki clan, and the ones on the full moon could be the minks or the lunarians; The thing about killing the sun god I guess is referring to Joyboy's defeat; the part about the sea god storming could refer to the sea cataclysm described by vegapunk.
third world: this is the one I find easiest to understand (or well, it's the one I feel most confident about regarding my theory). It basically describes a war on a certain day involving several races along with the return of the sun god, but Ripley says that giants have never fought side by side with so many races, so I think this part is not so much the current world, more like what's going to happen in the near future. It predicts the arrival of Nica (luffy) and his alliance/friendship with other races against a common enemy (imu).
Again, this is just my theory, and I'm not closed to the possibility of being wrong.