It does not mention “humans”, only the civilization of Adam.
No one was intrinsically righteous (except by faith says Hebrews) in Adam’s lineage from the beginning, but that wasn’t the reason for the flood. The reason for the flood is because wickedness had grown. They had become very evil, and they had the added burden of knowing the true God and still being evil. There is nothing in the story that makes us think those outside that area were equally evil or responsible. One can be unrighteous without being evil. Maybe they were righteous too.
The “intent after all” was to restart the civilization of Adam, which had grown too evil.
You think I’m being novel here, but there is a lot of precedence, both recent and ancient for what I am telling you. You might want to read: