Well, we’re not talking modern subdivisions, to be sure. But Noah had to have neighbors. Otherwise where did his son’s wives come from? Adds a whole new flavor to “no man is an island.”
But yes, you’re right, the tin foil hat business is not in Genesis. It’s a reasonable inference based on New Testament texts (Matt 24:38, 2Peter 2:5, etc.) saying that everyone but Noah acted as if life would go on as normal. Why would they do that if they believed Noah? And we know they did get an earful from Noah, because he is accounted as a preacher of righteousness. If his preaching followed the pattern of other prophets, he surely preached of God’s coming judgment, and that must be the important part they didn’t believe. An extra-Biblical Jewish tradition even recounts the words by which he exhorted the unbelievers:
“be ye turned from your evil ways and works, lest the waters of the flood come upon you, and cut off all the seed of the children of men.”
~Pirke Eliezer