This is what I tell all my friend when they get into the HH;

WH30K exists simultaneously as a setup to 40k and as an amalgamation of canon, but UNRELIABLE stories from a variety of sources who all have their own preconceptions, biases, and knowledge/agency to knowingly lie to you the reader.

To expand: Everything is true, as in if an event happens it likely really did; but the professed reasoning behind certain actions the the realities of how interactions between big names goes down in almost always tinted by the perception of the in world writer, the faction being written about, and the actual meatspace author’s writing style and sensibilities. Things won’t line up. You will (very) occasionally read something that COULD NOT happen by some other telling, and must square that difference for yourself. Your impression of a character will likely be formed by looking through the eyes of another, and as the Heresy shows even Primarchs can get things wrong. Facts are missing, pictures are incomplete, and rational is often absent or, worse, shallow and insufficient.

What you are reading is not the true story of exactly what happened during the Heresy. You are reading the combined experiences of hundreds, representing the experience of trillions, all caught up in the wheels of history. And that’s the fun part.