Good question, actually. I had understood the passage to mean that Adam and Eve were immortal until eating from the tree of knowledge, but now that I think about it that doesn't make much sense in light of Genesis 3:22. Seems more likely that it's equating innocence of death with freedom from death, which is a reasonably common motif in Abrahamic tradition; on its face, though, it's hard to deny that it's true at best from a certain rather tortured point of view.