> it can also connote the completeness of the judgment
But if you are trying connote a judgment that is more complete than it actually was then it would be deceptive. So for whatever reason the writer of the text wound up being deceptive about geography. I am assuming it was through ignorance because the writer didn't know such a flood was impossible.
And it doesn't really matter whether it was all mountains were underwater or just covered in some other sense. It goes beyond geography to include biology, genetics, physics, and so on. Things that the writers were not aware of because their worldview was so limited, as you have pointed out already.
?a theory proposed by Glenn Morton
That does not contradict my claim that a flood caused by rain could ever cover mountains with water in a local area. His theory involves a huge inflow of ocean water. It also couldn't be the flood referred to in the bible if he's saying it happened 5.5 million years ago.
> There's another possibility at least for our understanding
This possibility would not accord with the bible story in terms of location or the impact it would have on human and animal life.
>Except that this story is told in many separate cultures.
There are lots of myths that different cultures either borrow from each other or come up with independantly. Why is this one more believable? I would say to me it is less believable because there would be evidence if it had happened. Wheras something like dying and rising god myths are also common, but would not be expected to leave any evidence behind.