That happens almost every single time Luffy beats a villain from Croc to Lucci to Doffy and Katakuri, so no, he was not right. You don't get to see Luffy's true potential until the end of the arc.
Yes. Flowing Haki is Luffy gaining his "key". Against Crocodile it was water/blood, against Katakuri it was FS, against Kaido it's Ryuu.