The 'buyer' had to be in on it.
If somebody from the public got their hands on it they might take it apart and post videos on Youtube proving Banksy's video was a lie. Banksy can't allow that to happen so he has to arrange for a friend to 'buy' it.
Then good ´ol banksy might have a legal problem, imho it would be highly fraudulent to drive the price of his own works up.
I am sure you can buy your own work back at whatever value you are willing to pay, therefore it would not be a viable fraud in itself (there is no direct financial advantage in buying back at a higher price than market value) but at the same time the value of his other works changes price tags as well.
At the same time revealing the mechanism or the truth might be a legal way out of the problem -> to not influence pricing too much. Anonymous or not...
It would be like a stock manipulation by the owner of a company.... smoking weed in front of cameras, talking bullshit all the time and such things shortly after talking about buying back stock at a fixed rate above market value.
edit: after thinking about it for a few more minutes, i would say that this is an interesting discussion. Should such a case ever be in front of a court (forseeable for the artist), there are only a couple of options.
a) painting was destroyed: they get an expert for art that either concludes
a1) shredder is part of the artwork and worked as intended (nothing new in our little circle, see buzzwords "planned obsolescence", "right to repair" and such)
a2) painting destroyed, by common understanding a cut painting is a destroyed painting
b) painting was not destroyed, only a cut copy was rolled out, then the painting itself was unaltered (does the remote work in reverse?)
c) artists consider themselves not part of the society and see destruction as art in itself, with questionable outcome
Remind, banksy is known from the graffiti scene, in which point c) is often answered different by most people, i guess this is the root of the idea to actually alter something in a way people could see as destruction. So if the market value goes up, it would not be destruction, if it goes down it would. Good luck defining destruction based on market value now. I assume he has a good laugh on that one.