It is pointed out in the novel that they have a much greater goal than killing Guilliman.
"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.
The warp storm cut off the Ultramarines for years, leaving them unable to intervene or help at all until the very end...which is exactly what Horus wanted. But then again the preivous fluff you reference had the Ultramarines being attacked at the end of the Heresy, not at the beginning.
4/5ths of the Ultramarine fleet was gone along with at least a hundred thousand dead Ultramarines. In addition Lorgar and Angron's forces are also operating in Ultramar. They sucessfully completed their goals of putting a warp storm in place and caging Guilliman. They never intended to destroy him.



Wyrmwood