As Keith A. says....they didn't even know the earth was round in about 5000 BC. If your whole life is centered in an area of a few days travel, and you 'hear' of someone who has made a long trip, one BIG flood like the Bosporus event can easily lead to legends.
Suppose some guy managed to save his family and a few animals by floating on some debris, and later told stories about it?

Even the bible doesn't speak of 'the world' as a sphere, but uses words that basically mean 'everything we know about'.... which for most people wasn't much.