Does crazy Richtofen ever mention energy? Realize a scientist isn't going to even consider the idea of souls compared to energy and matter. We see this present-day as well. If a scientist mentions souls he's exiled from the left-scientific community (which is a large portion). Energy is more proper for them. Crazy Richtofen from WaW and BO would give us better thoughts, since he has been in direct contact with the MPD, but I don't believe he even takes time to mention it. I don't want to argue whether it's souls or energy, but I have a concern disregarding souls as well. Zombies weren't the only ones slain on Moon. It's widely believed, though I don't think specifically confirmed anywhere, that the original inhabitants of Shangri-la were sacrficed to the MPD as well. Are they just energy too? What about the rat?
And I wouldn't use the lantern as a comparison between the two. If I recall correctly, the wisps found throughout Buried and in the lantern are never compared to the energy or souls we absorb into the MPD or the eggs.
Also, Samuel hears voices in his head, but they aren't necessarily zombies. For all we know, they could be Keepers. And let's not even go into people arguing that it's Richtofen (I still firmly disagree that the unintelligible voices are him, though the last one is).
I won't argue that energy is definitely there and I don't think anyone wise would. But I don't think we can just disregard souls playing a part in this as well, much larger than just the transfer of one to another body in Moon.