Noah and his family and the animals on the ark are said to be atop floodwaters that are at least 20 feet taller than the highest mountain. This is probably for the sake of sounding believable (stifled laugh there), because already, for some reason, the dimensions of the ark have been described, and the author wants to make sure the story is cohesive.
The thing that the author(s) didn't know was that when you go very high up into the atmosphere, like when you're climbing Everest, you run out of oxygen. This is why some climbers (pansies) take oxygen tanks with them, and the others take their sweet time acclimitizing to the rarefied air.
The 'death zone' is the term used by mountaineers to refer to any height above 8,000 metres, or 26,250 feet, above sea level. The summit of Mt. Everest is 8,848 metres above sea level, or 29,028 feet. The reason for giving these altitudes such a buzz-killing name is that the air pressure is much lower and therefore your blood will only be partially saturated with oxygen. Your body will try to compensate for lack of oxygen by ramping up the production of red blood cells and increasing frequency of breathing and heart rate. This acclimitization should happen over a long period of time, and I think that forty days and nights would be a pretty good length of time for it.
The only problem is that humans (and many other animals) cannot live in the death zone (imagine that!) for very long. Your digestions shuts down, and you can't sleep, and all your bodily functions slowly deteriorate and then you die.
In contrast, the summit of Mt. Ararat is only 5,137 metres,16,854 feet, high. Twenty feet taller than this would be a walk in the park as compared to twenty feet above Everest. And it's likely that the author knew about Ararat, and unlikely they knew about Everest.