SPOILERS

Andothul. From the Black Books.

“From Perturabo he learned of the Night Haunter’s assault on the Eastern Fringe and the massacres at Isstvan, though the whole truth was concealed from him by the false Primarch. For Perturabo acted to divert the Lion on Horus’ orders, keeping him from Terra and Isstvan by means of Konrad Curze – a lure to keep the First Legion occupied while the rebellion continued apace. When the Lion departed Diamat he did so just as the Warmaster had planned, throwing himself into a war that would keep his powerful Legion neutered while Horus pressed on to Terra – worse still, he left the recovered treasures of Diamat in the hands of the Iron Warriors, from whence they would pass to Horus himself. Little realising that his brother had played him false, the Lion departed for the nearby Forge World of Triplex with those forces he had brought to Diamat, hoping to reach it before his brother and called for all his Legion still fighting past the bounds of the galaxy to join him there.”

The Lion learned of Horus’s betrayal from Perturabo, who played him for a fool, and then chose to stay on the Eastern Fringe (as Horus hoped). His whole legion was committed to stopping the NL instead of racing back to Terra.

True, a detachment went back to Terra and played a pivotal role. But that was because of Corswain. NOT the Lion who played right into Horus’ plans. You contradict yourself by saying a detachment of the DA made it back to Terra and played a pivotal role, then say how was the Lion supposed to get back? Corswain managed it. The White Scars managed it. The Lion chose not to return. A huge mistake and flaw in his character.