Well it's only a minor issue really. It may not even be explained fully.
If it is addressed then it would suggest that at some point during Secret Wars Hickman will be sorting out the whole connection between cosmic cubes and Molecule Man. Maybe even implying that the cosmic cube is part of the fuse mechanism. The juggling cubes have been pulled from the dead Molecule Men. Somthing seems to be falling out of Doom's box and I would hazard a guess that it wouldn't be a pile of corpses numbering in the billions for taste reasons. Billions of cosmic cubes would certainly make sense.
P.S. This issue may be all the implication necessary. The idea fits the events.