I don't think a flashback is needed for this particular type of Haki. Ray basically said Luffy could not attain it throught training but through strengthening his own will. Luffy having a 1 on 1 is actually good for his growth. I am a bit shook tho, Kaido is yet to get his flashback and awakening. it is about to be one of Luffy's roughest ride in the series yet but it would be totally worth it. This is already way more than I expected from him. Ad CoA and ad CoC all in one arc. I swr I am actually proud of that boy