I'm not sure why people argue that Oda planned Haki based on Mantra in pre TS, or Shanks scaring away a sea king. Not sure if people are just misunderstanding the topic, but no one is denying that Oda referenced those things when Haki was actually established in the story, but it's clearly not the original intention. Do you think he was writing chapter 1, with a chart in his hand at the same time listing the three types of Haki, and deciding that Shanks was using Supreme King Haki, instead of it just being a general anime aura moment, which plenty of series do? It's clearly not drawn in the same way COC is used today, with all the lighting and people passing out, unless you believe that Shanks secretly held back his COC on the Sea King, but at that point we're just doing rationalizations, which is a waste of time.
And again, how do you explain the legendary prodigy Rob lucci not having proper Haki? This is despite the fact that he was being glazed by vice admirals, and according to a relatively powerful warlord in Kuma, saying that no one expected him to lose, which is obviously ridiculous in retrospect. This is despite the fact that literal irrelevant new world bums all have Haki.