The difference is that Haki doesn't get tricked by "close to death". A voice being gone means that a person is dead. That isn't up for debate.
We have showings of Momo
seeing fading voices, so we know what it's like for a character to actually be close to death. It wasn't like that. His voice was completely gone. The fact that being alive produces a voice, and Luffy himself has a strong voice, his voice being gone means that he's not alive.
Haki thinking that he's dead means that he's dead.
Also, Kaidou doesn't say "I thought you were dead". He says "I was certain... that you were dead". This isn't "oh i thought you died", it means "from what I know, you died". Luffy, the owner of the body, thought the same thing too. This is why he killed the CP0 agent, because he knew he interfered and got his enemy killed.
Him "possibly" being dead is contradicted by everything saying that he actually was dead. People getting knocked out that bad has never been a thing of "oh he's dead" in all 1049 chapters of One Piece.