No, Tact, I certainly don't have all the answers. And yes, "incredible" would be a word. Though the Great Flood and its aftermath were an incredible events.
At this point we'd merely be speculating (yes even more so). Perhaps some subsequent deluges diluted and washed away salt remnants underground or beyond via long since-gone rivers and streams outlets leading to oceans. Or via glacial melt-off and breaches in inland freshwater seas during the Ice Age.
Then you will understand why I don't find them credible.