If something can be pierced ofc it can be destroyed what are you on about. An asspull binding vow had to be create dout of thin air to allow her to bypass this rule and affect the finger
Not necessarily. The fingers might heal/regenerate. Or displace, liquid-like when need be. Thus not able to be truly destroyed.
Edit: My paragraph above may be possible but not supported in-story. Chapter 267 page 9. It comes down to a binding vow of penetrating not “destroying”.
Sukuna’s primary weakness was his soul. He freaked out when Mahito touched his soul. What does resonance do? It affects souls primarily. Nobara vs Mahito was a major flag/foreshadow of what would happen vs Sukuna. Resonance affecting Sukuna’s soul is just logic.
Binding vows are asspulls. But jjk is built on them.