Here are two generic references in Book 5 that perhaps holds open the possibility that such stories might be true, but definitely doesn't say that they are:
QuoteDisplay More[324] Moreover, if there was no birth and beginning of the earth and sky, and they were always from everlasting, why beyond the Theban war and the doom of Troy have not other poets sung of other happenings as well? whither have so many deeds of men so often passed away? why are they nowhere enshrined in glory in the everlasting memorials of fame? But indeed, I trow, our whole world is in its youth, and quite new is the nature of the firmament, nor long ago did it receive its first-beginnings. Wherefore even now certain arts are being perfected, even now are growing; much now has been added to ships, but a while ago musicians gave birth to tuneful harmonies. Again, this nature of things, this philosophy, is but lately discovered, and I myself was found the very first of all who could turn it into the speech of my country.
[338] But if by chance you think that all these same things were aforetime, but that the generations of men perished in burning heat, or that cities have fallen in some great upheaval of the world, or that from ceaseless rains ravening rivers have issued over the lands and swallowed up cities, all the more must you be vanquished and confess that there will come to pass a perishing of earth and sky as well. For when things were assailed by such great maladies and dangers, then if a more fatal cause had pressed upon them, far and wide would they have spread their destruction and mighty ruin. Nor in any other way do we see one another to be mortal; except that we fall sick of the same diseases as those whom nature has sundered from life.
[351] Moreover, if ever things abide for everlasting, it must needs be either that, because they are of solid body, they beat back assaults, nor suffer anything to come within them, which might unloose the close-locked parts within, such as are the bodies of matter, whose nature we have declared before; or that they are able to continue through all time, because they are exempt from blows, as is the void, which abides untouched nor suffers a whit from assault; or else because there is no supply of room all around, into which things might part asunder and be broken up—even as the sum of sums is eternal—nor is there any room without into which they may leap apart, nor are there bodies which might fall upon them and break them up with stout blow. But neither, as I have shown, is the nature of the world endowed with solid body, since there is void mingled in things; nor yet is it as the void, nor indeed are bodies lacking, which might by chance gather together out of infinite space and overwhelm this sum of things with headstrong hurricane, or bear down on it some other form of dangerous destruction; nor again is there nature of room or space in the deep wanting, into which the walls of the world might be scattered forth; or else they may be pounded and perish by any other force you will. The gate of death then is not shut on sky or sun or earth or the deep waters of the sea, but it stands open facing them with huge vast gaping maw. Wherefore, again, you must needs confess that these same things have a birth; for indeed, things that are of mortal body could not from limitless time up till now have been able to set at defiance the stern strength of immeasurable age.
[380] Again, since the mighty members of the world so furiously fight one against the other, stirred up in most unhallowed warfare, do you not see that some end may be set to their long contest? Either when the sun and every kind of heat have drunk up all the moisture and won the day: which they are struggling to do, but as yet they have not accomplished their effort: so great a supply do the rivers bring and threaten to go beyond their bounds, and deluge all things from out the deep abyss of ocean; all in vain, since the winds as they sweep the seas, diminish them, and so does the sun in heaven, as he unravels their fabric with his rays, and they boast that they can dry up all things, ere moisture can reach the end of its task. So vast a war do they breathe out in equal contest, as they struggle and strive one with another for mighty issues; yet once in this fight fire gained the upper hand, and once, as the story goes, moisture reigned supreme on the plains.
[396] For fire won its way and burnt up many things, all-devouring, when the resistless might of the horses of the sun went astray and carried Phaethon amain through the whole heavens and over all lands. But, thereupon, the almighty father, thrilled with keen anger, with sudden stroke of his thunder dashed high-souled Phaethon from his chariot to earth, and the sun, meeting him as he fell, caught the everlasting lamp of the world, and tamed the scattered steeds, and yoked them trembling, and so guiding them along their own path, replenished all things; so forsooth sang the old poets of the Greeks: but it is exceeding far removed from true reasoning. For fire can only prevail when more bodies of its substance have risen up out of infinite space; and then its strength fails, vanquished in some way, or else things perish, burnt up by its fiery breath.
[411] Moisture likewise, once gathered together and began to prevail, as the story goes, when it overwhelmed living men with its waves. Thereafter, when its force was by some means turned aside and went its way, even all that had gathered together from infinite space, the rains ceased, and the strength of the rivers was brought low.