I see it as both Guilliman and the Lion getting in each other's way but also something else. I think that when Curze fought the Lion in Savage Weapons he wasn't at the top of his game, he wasn't in full control of himself. I think that Curze in The Unremembered Empire is him fully lucid and in complete control of himself, and thus fighting at his full potential. So... quite scary stuff.Though the Lion has proven that he is more than capable of fighting Curze in the past. I still see it as more, Guilliman and the Lion getting in each others way.