I think he was suggesting anyone can get a hold of your inkan and use it to sign things in your name.
I can go to the bank teller, offer them my driver's license, citizenship card and can recite the PIN code to them but they won't believe I'm me and let me access my money.
Show them my plastic stamp and I'm magically me! I could give it to another individual and he could withdraw money from my account, no questions asked.
Possession is simply not security.