Thats what luffy trained for and kaido suddenly busts out haki it makes it irrelevant.
Luffy just saw a glimpse of Kaido's abilities, he even failed to
injure him which is literally the
1st step as we can see about the Scabbards. Afterwards, he has to match his combat abilities as well - with forms like Bound Man and Snake Man, he should have it easy to connect powerful hits.
That's the part which won't happen that easy. Luffy is still the underdog and he just can't make him that powerful in front of the antagonist that early.
Kaido took a fatal blow against a Ryuo user 20 years ago. The same dude told him to get stronger. You think Kaido wouldn't use this opportunity to get rid of this weakness in front of skilled haki users?
Channeling his Haki through his weapon should be the first step to be stronger than before btw.