There is no doubt that the calculations are biased on the low side. The fact is without the calculations knowledgable people are aware that there isn't enough water to cover the earth even if you were to squeeze all of the H2O out of the atmosphere, glaciers, poles and mountain tops. Your argument doesn't hold water (forgive the common pun).
There is evidence in historical records that a great flood happened. Whether the evidence points to a single flood event is uncertain. But just for the sake of an argument, say it did rain for 40 days and 40 nights. The total rainfall would have caused massive flooding as the rivers and streams would not have been able to handle the deluge. Bridges and roads would have been useless if not destroyed. The only means of transport would have been by boat. The volume of water would have been so great that controlling a boat would have been highly unpredictable. A family trying to escape to a safer haven would have taken their children and animals with them if the boat was large enough. The lack of food and safe water would have caused the death of many people whom might have survived the flood and more would have died after the rain stopped.
It would be probable that a family able to survive a flood of this magnitude would have awaken one morning to find that there was no one else within the realm of the world they knew. So the story could be based on a great flood and the measurement of flood depth could be accurate. Whether it was over all the lands would be a subjective conclusion for it would have been highly improbable that they would venture very far in the conditions that would have existed after the flood.
The only question in my mind is whether it was possible to collect the number of animals that were supposedly on the ark. The reason I question this is related to their the ability to gather the number of animals in the far away places they would have to travel to round 'em up. That is where story falls apart. This would lead one to assume that the story is about a region, valley or some other location that may have been the world to them but not the world we know today.
While you might not agree with my thoughts, you might recall that Alexander is given credit for conquering the world. An yet he didn't step foot in Austria or the Americas. It would appear that sometime in the past the definition of the word "world" may have a lesser meaning than construed today.