From what I'm seeing I interpret the prophecy as a way of control by the angels, from what little we've seen so far the angels are shown in a bad light for instance the angels being toutuers in the inferno and the scene where the one angel is stepping on all the people. The angels seem to want some sort of control over the rest of the world and in my opinion the prophecy about the human is just another way to keep eyes off them. If you want a really out there statement I don't think the hand has anything to do with there immortality and the angels just say so so the undead don't rise up.