That's a common dogma. Another is that "day" didn't mean 24hours. Still another is that they died a spiritual death. There are others dogmas read into the story to make it say something other than what it actually says.
I also noticed he brings up "leviathan". Do you really believe this was some kind of real or mythical creature?
It begins in Job 40 speaking of the Behemoth and onto the 41st chapter speaking of the Leviathan. It's an interesting read with a much deeper message than a real or mythical creature. It's God speaking against the "pride" of man and describing it in poetic language which must be discerned, but the clues are evident within the text.
Problem with scripture is man attempts to place God on the same level of existence as us, for God is not tied to the time of our solar system and planet Earth. Then in Isiah it's brought up again giving us more clarity:
(Isa 27:1) In that day the LORD with his sore and great and strong sword shall punish leviathan the piercing serpent, even leviathan that crooked serpent; and he shall slay the dragon that is in the sea.
Comparing Leviathan to a serpent.....beginning to see what's happening? Go back to Gen and glean more clarity:
(Gen 3:1) Now the serpent was more subtle than any beast of the field which the LORD God had made. And he said unto the woman, Yea, hath God said, Ye shall not eat of every tree of the garden?
(Gen 3:2) And the woman said unto the serpent, We may eat of the fruit of the trees of the garden:
(Gen 3:3) But of the fruit of the tree which is in the midst of the garden, God hath said, Ye shall not eat of it, neither shall ye touch it, lest ye die.
(Gen 3:4) And the serpent said unto the woman, Ye shall not surely die:
(Gen 3:5) For God doth know that in the day ye eat thereof, then your eyes shall be opened, and ye shall be as gods, knowing good and evil.
(Gen 3:6) And when the woman saw that the tree was good for food, and that it was pleasant to the eyes, and a tree to be desired to make one wise, she took of the fruit thereof, and did eat, and gave also unto her husband with her; and he did eat.
Remember there are two trees in the midst of the Garden, the Tree of Life along with the Tree of the knowledge of good and evil. When asked of the serpent, Eve fails to mention the Tree of Life in the midst....a mistake or purposeful. In comes 'pride' spoken by the serpent and the description of the Tree of the knowledge of good and evil, subtly luring the woman to disobey God.