To be fair, decapitation would mean an intact head; and then stasis lock. An almost point blank blast of Optimus' weapons wouldn't actually leave a head, and probably take some of the upperbody with it too. Okay first thing, that post you quoted had that part in spoiler tags for a reason. Have the courtesy to keep the spoiler tags. His proof is the word of what I would presume to be the writers. It is possible they're just shooting shit, but I want you to try and follow this idea: Unicron is not a standard Cybertronian, or indeed even a being. Someone who can create avatars for himself out of the ground upon which you stand, and who's mere act of waking up to scratch himself causes catastrophic weather patterns is not to be underestimated. As I also said "Even a dead God can dream", we all know who said that and why.