Some people think that because what Boa Marigold said in chapter 517. But is she really a reliable source? To me it sounds like a rumor by an irrelevant character, and there have been shit-ton of rumors in the OP-verse spread around by people that were later proven to be false. She's a fodder compared to all the relevant characters and she doesn't really sound like someone who has deep knowledge about Haki or fighting in general. So why do we take her words for granted?
Also what Garp said shouldn't be taken literally as it could have meant something like "you're just like your father", not that Luffy was literally born with it. Exactly like what that Vice-Admiral also said when Luffy used it
When Rayleigh was explaining CoC to Luffy, he never mentioned that you have to be born with it, even though it's supposedly a pretty big detail
He only mentioned that the people who made names for themselves possess it, which could mean many things honestly. Maybe it starts to manifest by itself after the user consciously or unconsciously transforms himself and his spirit into a conqueror-like one, and the Conqueror's Haki would be the manifestation of said spirit, but no reason to think that you HAVE to be born with it even if some people like Big Mom or Ace managed to unlock it very early in life as they might have naturally possessed spirits compatible with CoC, like how Aisa was born with CoO because she naturally had it's spirit
Also what Garp said shouldn't be taken literally as it could have meant something like "you're just like your father", not that Luffy was literally born with it. Exactly like what that Vice-Admiral also said when Luffy used it
When Rayleigh was explaining CoC to Luffy, he never mentioned that you have to be born with it, even though it's supposedly a pretty big detail
He only mentioned that the people who made names for themselves possess it, which could mean many things honestly. Maybe it starts to manifest by itself after the user consciously or unconsciously transforms himself and his spirit into a conqueror-like one, and the Conqueror's Haki would be the manifestation of said spirit, but no reason to think that you HAVE to be born with it even if some people like Big Mom or Ace managed to unlock it very early in life as they might have naturally possessed spirits compatible with CoC, like how Aisa was born with CoO because she naturally had it's spirit