Banksy did acknowledge installing the shredder "a few years ago" in the event the painting "was ever put up for auction." In other words, he was playing the long game and likely hid a long-lived battery (possibly lead acid) to ensure he could destroy the work well after framing it.

It's not certain who bought the painting or how they'll react to the trick. As for the meaning? That's not clear, either. The shredding certainly garnered publicity, but it could have been a statement on the value of art in high society -- people are frittering away millions at auctions when it's the significance of the art, not the physical object, that matters. If that's the case, though, there's a degree of irony in that the value of Banksy's piece might increase in its half-shredded state. How often do you see era-defining art that's designed to self-destruct?