Baseless
Capone said Big Mom body is like an iron baloon that does not imply that Big Mom used armament to coat her body. Her body without armament is like an iron baloon. Iron has no relation to hardening or armament haki.
Fodders made it a big deal that Shiryu stabbed Garp because none of the fodders managed to hit Garp so when Shiryu hurt Garp the fodders saw it as an opportunity to kill Garp because Garp was weakened they could hit Garp now since Garp movement got slower due to the stab wound.
The author never refers to iron as garp armament defense. The author never drawn Garp barrier or his gardening no blackening it's all in your heads.
If Garp used barrier and still got overpowered this is the scene Oda would have drawn
If Garp coated his body with armament and still got pierced this is the scene Oda would have drawn
Blackening on Luffy forehead to imply Luffy hardened his forehead with armament to resist Rebecca's sword.
None of those happened during Shiryu and Garp scene.
Garp did not use hardening. Shiryu used hardening.
Why did Oda draw hardening on Shiryu sword but not on Garp's body? If Garp used hardening Oda could have drawn blackening on Garp's body since he drew hardening on Shiryu's sword but he did not. Oda chose to show blackening on Shiryu's sword but not on Garp's bodyGarp did not use hardening