"Calth is his operation, far more than it is Lorgar’s. Kor Phaeron has planned this for his primarch meticulously, and executed it with the aid of Erebus. The punishment and annihilation of the XIII is its principal aim; the humiliation and execution of the wretched Roboute Guilliman. But it is also an advancement, another step on the spiral path of the Great Ritual. It will allow their beloved primarch to progress.
Sorot Tchure is aware of his commander’s burden. There is no room for failure. There is a priceless and vital military objective to be won, but even that pales into nothing beside the greater intent."
Noted the bolded. At the end of the battle Erebus notes the true nature of the overall Word Bearer plan and is hardly concerned at all that they failed in killing Guilliman.