What does the Genesis account mean what it says "earth"?
We, today, think of "earth" as "the Earth", our planet, however the two ancient Hebrew words that could be used to describe the word "earth" were references to "dry land".
Much like how Genesis 1:10 reads, "And God called the dry land
Earth; and the gathering together of the waters called he Seas: and God saw that it was good."
The word "Earth" in the Genesis account meant "dry land", not necessarily the planet.
Also, how could the writer of the Genesis account possibly determine that all living things on the planet were killed or that the entire planet was covered in water?
Wouldn't any of his claims about the devastation of the Flood be bound to his limited perspective? He could only describe what he could see.
Also, what if the Garden of Eden and all subsequent events up to the Flood happened on an island?
You need to make a lot of assumptions to come to the conclusion that it was a world Flood.