If this is all there is to the matter, just Bucky's one-time remark and no one acting on it, I would just take it as him saying something wrong for whatever reason. Maybe his memory is screwed up. Maybe he was tricked or hoaxed or something. The Human Torch killing Hitler has been established too strongly already.
Seriously, the day Hitler died must have been the busiest mess of contradictions and alternate timelines since Atlantis sank. Sheesh. He was cloned just before the original was killed, a clone really died, his brain was removed and saved, his consciousness went into another body, one of his doubles was killed instead, he went to Hell where the Devil agreed to send him back.. there have been hundreds of Hitler-death science-fiction and fantasy stories.