The old ones are who created the eldar, not literal gods, but a race that was so powerful they wielded cosmic power. They disappeared 10s of millions of year ago, and you can still see how deeply their bio-engineered structure is still influencing their creations. However, in their absence, the eldar have replaced them with new gods, and had to adapt to how the galaxy is post war in heaven. The Orks are a fungus, that was created as a bio-weapon race by the Old ones. The war in heaven, which was old ones vs C’tan, cause the psychic/spirit/whatever aspect of the universe to be corrupted into the warp. That taint is what caused most of these splinter groups. However the whole structure of things is based of lineage, everything important is the result of, and/or practices eugenics of some sort. You can even see it in the structure like how Hive cities are organized. Just because there are splinter groups doesn’t mean this stuff isn’t that. If anything it leans into it because the biggest reason things splinter from the groups largely have to do with warp influence, which is what real groups, like the nazis, would call degeneracy. Also, if you look at groups like Nazis there was always infighting, fractional groups, internal rebels, etc. The existence of things resisting the system doesn’t mean that isn’t what the system is, and that it isn’t the preeminent concept at the foundation of everything.