All of Akainuโs magma does consistent damage though, so this argument doesnโt apply. Jinbe used water to protect himself from Akainuโs magma, and with Curiel we really donโt know the extent of his injuries, but he likely had Haki to protect himself as well.
With Kaido, Luffy has no Haki to protect himself but is still tanking serious blows from Kaido as indicated by the lightning effects of Luffy tanking Kaido, as well as the fact that we see building-levels of destruction on the roof, and we know building level is the max damage output of Big Mom and Kaido since they fought for two days but still couldnโt destroy the room they were standing in. In other words, defenseless Luffy is tanking them at their full strength and he isnโt even using Haki.