There was no physical death in God's creation until the fall, as all animals chewed grass and ate plants, no meat eaters!You cannot argue that "death" "in that day" was part of natural creation because Genesis 2:17 and Romans 5:12 clearly and explicitly repudiate that theory. So, you are forced to deal with at least the beginning of death with regard to Adam's person "in that day" that previously did not exist with regard to his person.
You can't say he physically died as Genesis 5 denies that.
You can't say he was cast into Gehenna that day.
So death in some other sense began that day with regard to his person. Do you deny the existence of "spiritual death" in the sense of spiritual separation from God due to sin?? For example, how can one be "alienated from the life of God" (Eph. 4:18) being physically alive?