I know. The Nurgle bit got to me because like you said, it has nothing to do with natural biology/evolution and the same with the Khorne bit.That is a really stupid story. No personal offense intended, Malus.
The hive "evolved" to defeat Nurgle's diseases? I call bullshit, even if it is in the codex. The whole point of Nurgle's diseases are that they aren't natural. Evolution doesn't help. No "natural" disease causes you to become massively hungry and explode into mounds of nurglings. No "natural" disease kills you and turns you into a warp animated zombie.
And suddenly Khorne doesn't want skulls and blood (ichor) because they're from tyranids? Again bullshit. Khorne cares not from whence the blood flows, only that it flows.
The other difference is that a single world 'out there in the galaxy' isn't the same as a world in the eye of terror. The entire fabric of reality is bathed in the empyrean in the eye of terror. I suspect the effect of the shadow in the warp would be much diminished by the ambient and really available power of chaos.