IndianaJonny said:
I think that poster meant the whole "loyalist traitor" thing. Basically during the Horus Heresy an alien seer foresaw two outcomes from the Horus Heresy and informed the Alpha Legion of the two choices they had.
One was the grim reality that 40k is today where untold trillions are massacred or brutalized in an eternity of war where humanity clings to its half life. Humanity is destined never to grow ascendant again but instead to wither and die jealously guarding its territories and bickering amongst themselves. This would come to pass if Alpha Legion stayed loyal.
The other possibility was an unthinkable decision. If the Alpha Legion turned traitor and fought against their sworn brothers there was the possibility of a bright future. In this possible future humanity goes out with one big bang, Horus strikes down the Emperor and every human is killed but in their death throes the Imperium takes all the Chaos Gods with it into non-existence. All the other residents of the galaxy get to live in a universe akin to pre-chaos times where souls can reincarnate back into new bodies when they die, fast travel has no dangers, no one needs fear for their souls and the warp is a calm place.
That is why their warcry is "For the Emperor!" for they are the most loyal of the loyalist. They chose a great sacrifice in the hopes of making the galaxy a greater place. Unfortunately fate had other designs and the actions of others caused the bright future not to come to pass. Namely Horus' overconfidence/hesitation in the siege of Terra when he lowered his ship shields which allowed the Emperor and Sanguinius to teleport aboard. Basically Horus screwed the plan by seeing what he had become at the last moment and lamenting.
Their sacrifice was all for naught and no one knows.