Leviathans are whales....mammals that still exist today.
"Behemoths are large land creatures."
The only reference to a behemoth is in Job 40:15 which is describing a hippopotamus.....mammals that still exist today.
On what basis do you conclude that Leviathan is a whale?
On what basis do you conclude that Behemoth is a hippo?
OK, the iron age started around 1000 bc, how did anyone know about or understand iron before the flood (per answers in Genesis, circa 2300 bc). This could not have been written before the flood, iron was unknown and the bronze age was starting concurrent with the flood at 2300 bc.
Therefore, this description had to date to no earlier than 1000 bc and could not be a contemporary account of the creature.
Good night.