It's also fair to presume the Sumerians' ancestors survived at least one and maybe several "Great Floods" besides those they encountered in Mesopotamia.
Actually, the Sumerians and their Saami relatives might well have been among the few human beings to survive the melting of Antarctica. When that happened ocean levels rose hundreds of feet drowning all human habitation in our natural range ~ river estuaries!
What is the current thinking on the time frame? A slow melt wouldn't do the job. A rapid rise in sea level would, I presume, require that a massive continental ice pack broke free and slid quickly into the ocean. Is there evidence for this?
There is no evidence Hercules was not a historical figure either. Do you believe in Greek mythology as fact? Myths teach truth, not facts.