That's not a flood myth. Cause the cataclysmic flood was far away, and the ymaryn learned the 'flood' was a natural phenomenon they could stop. Also the reason the ymaryn having a source moment for a flood myth, isn't important.
What is important is if the Ymaryn never had the flood myth passed down. Look the 'flood myth' isn't something repeatable cause a city is flooded. We're talking civilzation is drowned in all of Asia flood, so long ago the story is a part of all religion. I'd like to imagine the nile floodings were common enough a flood myth wouldn't be the result.
The 'flood myth' cause the ymaryn had high prestige would be ignored by those who took the ymaryn religion up, or cannibalized the ymaryn religion. The ymaryn never saw the 'flood' as an unavoidable spiritual curse, so veekie can't use a moment in the past as justification for a flood myth due to cultural memory not valuing a minor flood as a disaster. Other places in the world would be similar, not having a flood myth. Not Austrailia might have a flood myth, but that seems unlikely unless the not hung nearly got drowned in the past. Which can imply the flood myth never spread around the world (unlikely due to AN's limitations on the butterfly effect, ex: the world map is a near-replica of today's landmasses and ignores natural weather disasters that shaped the past. OF COURSE A WORLD WIDE FLOOD MYTH EXISTS, cause rl writes the plot of history).
Realistically in PoC a worldwide flood myth can't exist, due to the whole flood real, real, bad, stigma never happened to spread in story to all civilizations in PoC.
None of that matters when AN needs the flood myth in all civs to play the rl analogs they have here. Flood myth is a cultural thing that is a piece of the heritage in rl, and a butterfly with unknown repercussions if crushed at the dawn of civilization (as occured in PoC).