First, it was said that there are several types of COA, and we ourselves know this.
First we see Luffy's hand which is completely black. Then Luffy tries to cover his body with more Haki and we see a white halo around Luffy's hand.
This is the advcoa which prevents contact
As is the non-contact punch
As we can see in this description, he is hitting without contact
And even if this image didn't exist, it wouldn't be hard for people with common sense to understand this
Luffy has already learned to defend and attack with coa, so what would learning advcoa add to him?
Yes, defending and attacking without contact
And the most advanced type of COA, you insert your Haki into people's bodies and somehow damage them from the inside, and we see that there is no limit to contact, and Luffy has touched it.