IIRC correctly Lion basically told the Chaos Gods to shove it (when speaking to Fateweaver), so it basically puts paid to the whole loyalist=traitor theory...

PROLOGUE
The Dead and the Dying

SKANE WAS THE ONE to find the body. Skane, armoured in Destroyers’ black, his armour stained by the sin of the weapons he wielded. He stood knee-deep in the dead, next to the wrecked hull of a Land Raider battle tank.

“Kargos,” he voxed. His voice was tinny, laden with static. One of the of the enemy had caught him in the throat during the battle, and it had jarred his augmetic vocal chords. They needed tuning once he returned to the Conqueror.

“Kargos,” he said again, across the quiet vox channel.

“What?” His brother’s reply was also flawed by static, but from more traditional vox-corruption rather than a bionic oesophagus.

“Track my locator rune,” said Skane. “Get over here.”

“Look around you, sergeant. You think you’re the only one that needs my help at the moment?”

Skane didn’t bother looking around. He knew where he was and what he’d see – he was at the heart of it all, and the dead numbered in the thousands. Most wore armour the green of shallow oceans, cracked and shattered by the treachery of their former kindred. These were Horus’s former Sons, betrayed by their brethren and slain for their disloyalty. Among their number, armour of bloodstained white stood out like pearls among seaweed. Too many World Eaters had fallen here, though victory was undeniable. The city was dead in every direction, reduced to ash and rubble.

A shadow fell across Skane, blocking out the weak sun as a Legio Audax Warhound passed with its rattle-clank stride shaking the tortured ground. He lifted a hand to the passing war machine, receiving no acknowledgement beyond dull sunlight glinting on the Titan’s ursus claw spears. It stalked onward, splayed feet grinding ceramite and bone and twisted iron into the earth, its wolfish cockpit lowered as it hunted for life signs and scanner-scents among the dead and the dying.

Skane turned back to the ruined tank, kneeling by its front end where the minesweeper plow was decorated in scratches and a wealth of gore. A body impaled on the ‘dozer blade’s spikes twitched in uneasy repose, its fingers still scraping in futility across the metal. Skane wasn’t sure how the pinned warrior still lived, and doubted the trembling, bleeding figure would survive being pulled from the plow. Nevertheless, he spoke again.

“Kargos,” he said for the third time. It took the Apothecary several seconds to answer.

“I told you I’m busy. Fix your own damn throat, or shut up and wait until we’re back aboard the ship.”

Skane disengaged the seals at the dying warrior’s neck, lifting the helm free with a hiss of released air pressure. The revealed face was pale, bloodstained from the lips down, the eyes open and blind, while the mouth worked in silent, wordless rage, an emotion lost between fury and pain.

“I’ve found Kharn,” Skane voxed.

This time, there was no delay in Kargos’s reply. “I’m on my way.”