The layer of atmosphere would be thinner, though, because of the increased surface area of the planet. But probably not enough to matter.
Compared to the much more significant problems (beginning with, where did all that water come from and where did it all go?) with the flood myth, being able to breathe at that high of an altitude isn't really even a problem.
The reason the ancient Hebrews believed it is because their cosmology held that the universe was basically water, separated from the earth by an invisible firmament. Considering how ignorant they were, this is not surprising. However, there's no excuse at all for modern-day people to believe that same flood myth, considering that we know that the universe is mostly empty space.