The four pages I'm about to show you are consecutive and from the same chapter.
Luffy palm strikes Caesar (a Logia, in an intangible Gas form) with Jet Pistol.
The very next page. Caesar is visibly hurt. He is bleeding. He says, out loud, "Aah, that hurt". Luffy was using Armament Haki. This is obvious, even though you can't see any coloration. He's just using basic
coating to hit the Logia user.
Caesar responds, and the situation escalates after his burning sword sets the room on fire. Luffy gets some air, then specifically calls out the use of Armament Hardening, a
technique that uses CoA
infused into a limb, or object, to physically make it harder and have more mass to result in a stronger impact.
Caesar can
fucking see it because its obviously black, and panics, because this is obviously a different technique that will hit him even harder than normal and maybe knock him out. Monet blocks just in time.