Brian, you made a mistake in English comprehension... ...If God intended "die that very day" then it would have been written "for in the day that thou eastest thereof is the day that thou will surely die".
Actually, Brian didn't make an English mistake at all. You seem to have overlooked that pesky little word "in," which switches the phrase "the day that thou eatest thereof" from a subject into an auxiliary clause modifying the verb "die."
"In the day that thou eatest thereof, thou shalt surely die"
...can also be written...
"Thou shalt surely die in the day that thou eatest thereof."
In modern English, we would more likely say:
"You will surely die on the day you eat it."
If Brian made a mistake, it certainly wasn't a mistake in reading English.