To be perfectly clear, Luffy was in base when Hody’s bite him. Here, I’m comparing Luffy’s haki when he is in G4 snakeman.
It is in this form that his haki is the stronger because it is in this form that is CoA is black/red. Which means that at full strength, Luffy’s CoA is black/red but Luffy (or any CoA used for that matter) is not always fighting with his CoA at full strength.