More to the point, many cultures preserved some kind of folkloric version of a great flood event (and the idea of this coming from the bursting of a great ice dam somewhere in Eurasia does seem most likely), and after many dozens of generations some of these flood stories wound up incorporated into much later, otherwise unrelated religions.
Exactly my point. Many unrelated religions incorporate the flood story because it was a universal experience caused by the sudden end of the ice age. Nobody “stole” any “myth” from anybody else.