Presumably, Banksy knew well that the stunt would increase the value of the drawing. So on some level, non-naivety is likely no matter what.
That said, it's still a commentary on the "art world." The unidealistic ways & reasons art is bought and how the value of art works... contrast to the idealistic artist destroying his work rather than having it denigrated. It's also inevitably hypocritical because the artist is either benefiting from the whole hype and hokum or wishes to be.
My guess is that the last part is why this is such a common navel-gazing motif in art. The heart at odds with itself. Idealists always struggle with all the hypocrisy, especially their own. Poor things.
I doubt that what came out the bottom was the original. I’ll bet that the exhibit can be rewound and replayed.
I doubt that's what Banksy would have intended though. Wouldn't surprise me at all if he just actually shredded the work.
Also, the story of the shredder being prepared years in advance reeks of bullshit. And if that part of the story is bullshit, I can't help but question the rest.