Consider the source. Lorgar was a primitive at the end of the day; he saw the Emperor in that context, because that's the context he had for these things.Because, as Lorgar puts it in First Heretic, the Emperor is a god in all but name, whether he admits it or not.
Well, that's kind of the point. By defining something as a "god", it says more about the individual's desire to worship than it does about the sufficiently powerful being. I've been watching a lot of SG-1 recently and this comes up a lot; I'm very much with Teal'c, as he consistently calls out the Goa'uld - they're not gods - they're just very powerful individuals. He refuses to worship them, and so doesn't call them gods, normally just Goa'uld.The Primarchs are awesome beings, the mere sight of which leads people to fall to their knees and weep and they are pale reflections of the Emperor. It falls down to how you define what a god is in that universe (the Eldar worshipped the Old Ones, were they gods?).
I mean, you've also got to consider that the Material Plane trumps the Immaterium every time: no emotions/psychic thought? No warp or Chaos Powers. Simple as. For all their villainous crowing about how they are the most powerful thing, the Warp exists because of people, not the other way round.