Well you seem very confused.
1. Seven times you have erroneously used the word "myth" to desribe the historial account of the global flood which occured Noah's day.
2. The flood account is not absurd or impossible or dogma or weak or easily discounted.
3. Eight people survived the flood, the ancestors of all human civilizations. "And God went on to bless Noah and his sons and to say to them: “Be fruitful and become many and fill the earth." ..From these the population of the isles of the nations was spread about in their lands, each according to its tongue, according to their families, by their nations.. These were the families of the sons of Noah according to their family descents, by their nations, and from these the nations were spread about in the earth after the deluge."
4. You are again misinformed. Noah's family was not incestuous.
5. "Don’t flood myths in other cultures actually count as evidence AGAINST the Noah’s Ark myth?"
It counts as evidence that such an awesome catastrophe, if it really happened, would never have been completely forgotten. Hence, in many nations there are reminders of that destruction. Practically all ancient peoples have a legend that their ancestors survived a global flood. African Pygmies, European Celts, South American Incas—all have similar legends, as do peoples of Alaska, Australia, China, India, Lithuania, Mexico, Micronesia, New Zealand, and parts of North America, to mention only a few.
Over time the legends have been embellished, but they all include several details indicating a common source narrative: God was angered by mankind's wickedness. He brought a great flood. Mankind as a whole was destroyed. A few righteous ones, however, were preserved. These built a vessel in which humans and animals were saved. In time, birds were sent out to search for dry land. Finally, the vessel came to rest on a mountain. Upon disembarking, the survivors offered a sacrifice. The similarities cannot possibly be coincidental. The combined evidence of these legends corroborates the Bible's ancient testimony that all humans descend from the survivors of a flood that destroyed a world of mankind.