Yeah, Hammond killed Hitler in April '45, a week or so after Cap and Bucky were blown up on Zemo's drone plane. IIRC, the Torch having killed Hitler was originally a Golden Age story, Young Allies or All-Select comics I think, which was made part of modern Marvel cannon by Roy Thomas. Hammond being the one who killed Hitler has been occasionally referenced in various Hammond appearances in Avengers, Namor, Heroes for Hire and New Invaders, and the Original Human Torch mini series that chronicled his life.
Maybe Bucky killed one of the Hitler clone Hate Mongers that Arnim Zola created during his Winter Soldier days. Either that or the writer simply didn't bother to do any research. God, I so miss the days when the editors at Marvel gave a damn about little things like this, but I'm sure the editors didn't even blink when they read this.