The simple solution to resolve how this was done is for the new owner to have a professional person to dis-assemble the frame and determine exactly how it was done. If the new owner is legitimate, and if they paid over a 1 million pounds, I would think that they are serious, we should have an answer in short time. At any rate, I smell bullsh*t.
Opinion time: I agree with Dave that this was done recently and those blades in the Banksy video are only meant to fool the most gullible among us. The blades were not used to shred the canvas. I also noticed the difference in planes of the actual painting and the part coming out the bottom of shredded canvas. Therefore, IMO, this could be a pre-cut canvas and the actual canvas is still rolled up on a separate roller.
If we do not hear from the new owner with a video or some other evidence showing this mechanism being dis-assembled, then the conclusion that I would come to is that the new owner is Banksy or a co-conspirator.
The fact that everyone is talking about this is probably what was intended, read: publicity stunt. Although it is humorous, it is just another run at the system to get attention for Banksy. (narcissism)