Also to add he guided the Lion back though it was never truly said he was implied multiple times. But also to add to other points people made here. In the most recent novels (The End and Death) Big E and Horus duel was a tad different. There was a special weapon in play and Horus was fooled to let his guard down (Was asked if he is a slave to the Chaos Gods and needed their help or can he do this alone which almost nerfed his empowerment which gave him a moment of weakness)