They were fighting based on Zoro's wordings on how they are invincible when flames are on, except that Zoro didn't know they can actually be damaged with adcoc due to lack of knowledge, so I don't think Luffy was using adcoc.
I've always really disliked this argument. Like, why would Luffy punch someone, realise it wasn't doing anything, and then continue punching them with the EXACT SAME FORCE, rather than just.......using stronger Haki. Even if you want to assume he doesn't START every fight using aCoC (even if there's no reason not to use it), at some point you would think he would stop being a moron and TRY it. Lol.
It is basically just arguing that Luffy is brain-dead and isn't using an ability he fully understands how to use for the fuck of it. 😅
At least Zoro has the excuse of being canonically slow and unlocking aCoC in an unorthodox way without realising thats what he did. Luffy did so consciously, he observed Kaido, realsisd what he was doing, and used everything he had learned about Haki to try and replicate it.