Hmmm, I don't know...
Given, they could have just been fundamentally dishonest, but
The Cabal's prophecy featured in Legion
is based on the assumptional observation that chaos was so heavily invested in humantiy, that the utter extinction of humanity (that apparently Horus would have brought about if he triumphed in the Heresy) would bring about it's destruction as well. However this complete investment is suppossed to be at the time of the Great Crusade/Heresy, it doesn't automatically mean that was the case from the beginning.
There are several established affairs which already refute humanity being the sole or primary contributor to the rise of chaos, and also refuting the Cabal's theory that the extinction of humanity would result in the extinction of chaos. Off the top of my head:
- The War in Heaven is what resulted in the warp becoming chaotic and implied to be when the chaos gods began to coalesce; '...The growing pains of the young races threw the untapped energies of the warp into disorder. War, pain and destruction were mirrored in the bottomless depths of the sea of souls. The maelstroms of spirits unleashed in the carnage coalesced into the previously formless energies of the warp. Older warp entities became terrifying predators, rending at the souls of vulnerable psykers as their environment was torn asunder and reforged by the energies called forth for warring in the material universe.' - Codex: Necrons, page 26.
- and that occured ~60,000,000 years ago according to our chronological perspective.
- We know that the vast majority of mortal species in the galaxy already feed and empower chaos, thus I fail to see how the extinction of humanity would bring about the destruction of chaos. It would certainly weaken it's influence, but destroy it? No.
- Slaanesh was primarily 'created' by the Eldar race, for chaos (thus including Slaanesh) to be solely invested in humanity a mere two hundred years after the Fall is absurd from this perspective.
We also have several passages in much more recent lore which seem to refute the chaos gods solely being human gods, or brought about primarily by humanity. Two easily found examples are:
- 'While one mortal lies, while envy and ambition survive, Tzeentch will work his magic as the puppet master of the universe.' - Codex: Chaos Daemons, page 12. The same can be applied to the other gods and their respective emotions.
- '...For billions of years these tides and waves flowed unceasingly through warpspace, and such is their power that they formed creatures made of the stuff of warpspace. These instinctual, formless beings gained rudimentary consciousness and the chaos gods were born - vast psychic presences made of the dreams and nightmares of mortals. As the races of the galaxy prospered and grew, so too did their hopes and dreams, their rage and wars, their love and hatred, feeding the chaos gods and nurturing their power. Eventually, the gods reached into the dreams of mortals their praise and servitude. A chaos god can only grow in power through the actions and thoughts of mortals.' - Codex: Chaos Space Marines, page 8.
- Note that it apparently took billions of years for the energies of the warp to coalesce (obviously way before humanity was around), and it clearly states 'mortals' rather than simply 'humanity'.
And of course there is always the strong possibility that the Cabal were lying to Alpharius, which (if I am correct) is even subtely hinted at in The First Heretic
Obviously it's possible that chaos is invested in humanity more than any other species currently ('...Chaos would also be much diminished without the rise of mankind.' - Codex: Chaos Daemons, page 23
) - but I think it's absurd to suggest that the utter extinction of humanity would result in the utter extinction of chaos. I also think that given the now known nature of chaos, we know that the chaos gods weren't a result solely of (or even primarily of) the rise of humanity. There were countless psychic races around long before the rise of humanity which would have contributed a lot more to the rise of the chaos gods.
(Think I may have drifted into an off-topic rant there, but oh well