I only claim you don't understand the world that's involved. If you want to talk about smoking Cuban cigars, start smoking Cuban cigars, you know?
*
The honesty of Bansky's remarks is akin to Artist's S* by Piero Manzoni.
The concept is pretty self-explanatory, however, Manzoni instructed that the cans should never be opened, lest they lose their true value. The cans are made of steel and can't be x-rayed.
Bansky is deliberately or accidentally making a throw-back to this and similar pieces of art (or "art" depending on how you feel about it). The mystery of the contents or intent is part of the experience of owning the Bansky or the Manzoni.
A single can of this **** sold for 124,000 Euro. A fellow artist claimed that these can contain plaster, but we'll never know. Similarly, the amount of destruction that was intended is part and parcel of the experience (thus value) of owning the Bansky.
If the Bansky was completely destroyed, Bansky could say that it was only meant to shred half the painting. I don't think either situation would alter the final intent or value of the painting. There is no logical reason to think otherwise.