I think he wrote that in jest...
Anyways, my favorite secret of the 40k universe has to be the emperor without doubt. It boggles the mind to even try to get a basic grasp of what kind of plan a being such as the emperor has been persuing for countless millenia, and who knows, perhaps even longer. Do we actually know anything about the old ones at all?(yes im of the belief that the emperor is an old one, that whole shaman story could perhaps be a group of old ones merging to create a super duper old one in the name of survival or something along those lines).
I have to say I also love the alpha legion too, that not only is there a second primarch in secret, but also that both primarchs could still be operating still is just awesome!