G4 increases his defence to both blunt as well as cutting attacks. Besides, Doffy's athlete thread is a blunt kick followed by strings, not a piercing attack. Doffy's kick bounces off.
How will Luffy's hardening magically get better just by blowing air in his muscles, please explain? It makes negative amounts of sense, haki comes from the spirit, not from the muscles.
I will prove to you that Luffy's hardening does not get better in G4.
1. Base Luffy, using hardening, clashes with Katakuri's Block Mochi fist and hurts his hand, showing Kata's superiority:
2. Luffy, later on, goes G4 and clashes fists with Kata's block mochi and hurts his hand again:
So you see, Luffy's hardening does not get better by simply blowing air in his muscles. G4's superior defence is due to the combination of his hardening and his rubber properties, as Doffy explains.
We have seen similar examples such as the above where Kata's block mochi combines hardening and his mochi, boosting his hardness.
And also Jinbei's shark skin armament that he used to block Cognac.