I have heard it theorised that instead Alpharius was wounded by Guilliman, who was then killed by Omegon. Alpharius was then disguised as Guilliman, and shipped back to the Ultramarines to super-infiltrate. Unfortunately the Ultras were unable to heal him, so he ended up in the stasis field. No idea how plausible this is though, I just like the idea that the Ultras are worshipping the body of their mortal foe. Does any book detail the battle? Were there any witnesses?