If If you attack someone and you don't hit them for whatever reason it's called a miss. It's irrelevant if you miss because they evaded the attack or if you have bad aim.
Has some of you never played dodgeball, evade your opponent's ball and then yell out that they missed? Seriously.
Kaido is not slow, he just a big creature which means he has more mass to move.
Luffy was right in front of Kaido's face, aiming to hit him in his face and missed, his face, because Kaido was fast enough to move his head out if the way. Luffy hit him just above his scar.
Yes he did get hit. Why, because he decided to move his body up instead of going down. If you would have went down Luffy might have completely missed. Luffy still didn't hit Kaido where he was trying to hit him at.