Well considering the latest interglacial (when the ice sheets started to retreat) period started just 11,000 years ago, it’s not surprising that many cultures have flood stories.
When the ice melted, the sea level rose, and coastal cultures around the world would have been affected. The Bering Strait flooded, the Black sea flooded, numerous islands sank beneath the ocean...
Yes, it is theorized that the central US was a low ocean, held back by a glacial wall in the rockies. When that broke, the water flowed out to sea.
You are correct, all we have is theory, stories and a few scientists' models....and we all know how reliable those are!