Remember, when reading this part, that humanity was growing, domestication (clean animals) and agriculture kept people to river beddings and the ‘world’ of every man/woman differed. It was as big as the horizon of the farthest distance they would walk. This often wasn’t much further than the next town. The animals they knew of, were the cattle and dogs or other domesticated animals they knew. Perhaps even the horses/camels.
An ‘ark’ didn’t need all of every animal on the face of the Earth. Just on the face of one’s world. And that was the farm they held.