Ironically, Games Workshop and Black Library in their infinite contradiction matrix did have one lore source say that it was the Warhounds that save Angron and left his fellow slaves to die rather then the GEoM.
The source:
As Captain of the Legion's 8th company, Kharn earned glory for himself as the War Hounds conquered planets and systems in the Emperor's name. However, it was not until the Legion were reunited with their primarch Angron that Kharn's true rise to power, and his descent into damnation began.
.......
When the Warhounds discovered Angron they rejoiced at being reunited with their Primarch, taking knee before him and offering up the leadership of the Legion. TO their dismay, Angron, having little interest in the crusade or the Imperium, turned his back upon them. In the end, the Legion forcibly teleported Angron onto their battle barge, sealing him in a cargo hold.
Convinced that they could change the Primarch's mind the captains and the commanders of the War Hounds entered the hold to make their case. One by one Angron killed them all, until Kharn - as one of the few officers of his rank - was the only one brave enough to face the homicidal giant.
Into the chamber spattered with the blood of his brothers Kharn walked, calm and composed, never flinching even as the looming shadow of Angron rose up from where he sat upong a pile of power armoured corpses. Angron attacked Kharn as he had the others smashing him to the ground and hammering him with furious blows.
However, Kharn did not defend himself, remaining in control of himself even as his instincts screamed at him to fight. In the end it was his calm bravery that won Angron's respect, and allowed him to convince the Primarch that master over the Legion was not a collar to be worn over his neck, but a mantle of power to be worn on his mighty shoulers - a mantle that would grant him the freedom to stalk across the stars in an endless war of glorious bloodshed.
-Warlords of the Dark Millenium - Champions of Chaos, pg.138 (mobi)
