I think it's much more insidious than you think. The raisin is symbolic of eris's withering allegiance to the vanguard. She was literally left down in the pit of Crota to be sucked dry of her light much like a grape is left in the sun to be sucked dry of its moisture to become a raisin. The box signifies the pit and only after it is opened can a guardian enjoy the delicious chewy goodness that is a raisin.... Or it could simply be a funny jab at eris that any 3rd grader would recognize without the joke having to be explained to them :)