The Chaos entity was torn, shredded, and assimilated in a way no different from real food. Everything was happening in the Mindscape so the significance of the event was clear.
Vastor wasn't fusing with his Abomination half nor was he asking to borrow its strength. The Master was consuming the rogue part of his body, destroying it so that where once there were two now there was one.
The Vastor-Clone struggled to break free, yet he failed. The Abomination was a feral beast, hungry for power for power's sake. He had no goal or ambition aside from becoming the strongest on Mogar and turning the entire planet into his plaything.
His body coursed with the power of the Eldritchs and through Vastor's memories he had the same magical expertise as the god of the battlefield. Yet none of those things belonged to the Abomination.