We must remember when reading about the flood in the bible what was their understanding of dry land or earth at that time,, before there were science, telescopes, Columbus discovery North America.
They certainly had no idea about gravity: up was up and down was down the stars were floating on a big circular dome. Galileo discovery that the earth was revolving around the sun which lead to Inquisition trail from the rulling Church was still yet 2000 years future.
So when it says the water overwelmed the highest mountain they weren't talking about MT Everest, they simply were making shit up and had no idea what the earth really was (definitely not a globe floating in space),, but dry land was all they thought it was and used their imagination to fill in the blanks of their understanding.