Why do Atheists say that the Noah's Ark story was stolen?

Some Atheists will tell me that Noah's Ark was copied from the Epic of Gilgamesh. From what I have read, the Sumerians descend from Noah and the Hebrews come from Shem. It is likely that Genesis was ped down to the Hebrews. Utnapishtim was the ancestor of Gilgamesh and if Noah is Utnapishtim this makes them related. Genesis is nothing more than rewritten rephrased and reintepreted doents from the Sumerians. So which is it? Are atheists just contentious or are they ignorant? Atheists, feel free to answer.