Because simple hardening should make his rubber like iron. But he retains his rubberiness. It's clearly more tightly integrated than just "layering" or "stacking" Haki on top of skin.
Doffy explains it "rubbery... in an armament state?" meaning he shouldn't be rubbery once Haki is applied, but he figured out how.
Doffy explains it "rubbery... in an armament state?" meaning he shouldn't be rubbery once Haki is applied, but he figured out how.
And besides, it doesn't prove that Luffy is using a new type of haki by doing that.