Myth or not, and despite what you believe it to say, the book does clearly say God told them right from wrong.
Gen 2:16-17,
16 And the LORD God commanded the man, saying, Of every tree of the garden thou mayest freely eat:
17 But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die.
Read that again. It's not a right / wrong question, it's a warning, like, Don't run through the briars barefoot.
Moreover, the text is unambiguous about when Adam and Eve gained knowledge of good and evil: AFTER and AS A RESULT of eating the fruit. First, the snake correctly informs Eve (Genesis 3),
4 'You will not die. 5 For God knows that when you eat of it your eyes will be opened, and you will be like God, knowing good from evil.'
And what happens?
6 [...] she took the fruit and ate; and she also gave some to her husband, and he ate. 7 Then the eyes of both were opened [...]
So up to that moment, AFTER they'd eaten the fruit, they had NOT had knowledge of good and evil; and thus when they ate the fruit, they were incapable of forming an intention to sin; and thus they were incapable of sin.
Moreover, nowhere in the Garden story is there any mention of sin, original sin, disobedience, the fall of man, death entering the world, spiritual death or the need for a redeemer. They're never mentioned. (They're an attempted retrofit from later centuries, but if you read the text, a failed one.)
Moreover, God is very specific as to why [he] drove them out of the Garden, and it has nothing to do with sin, disobedience, and so on: [he] says (and this is the ONLY reason he gives):
3:22 "Behold, the man has become like one of us, knowing good and evil; and now, lest he put forth his hand and take also of the tree of life, and eat, and live forever"─ therefore the Lord God sent him forth from the garden of Eden.
(That also directly contradicts any notion that death entered the world as a result of eating the fruit. Instead it says that Adam and Eve were always going to die.)
Also, if you read Ezekiel 18 you'll see that there can be no original sin, that sin can't be inherited: to take just one example:
20 The soul that sins shall die. The son shall not suffer for the iniquity of the father, nor the father suffer for the iniquity of the son; the righteousness of the righteous shall be upon himself, and the wickedness of the wicked shall be upon himself.