OP's method relies on the permutation of elements in (columns or rows) to be in the same order for each (column or row).
If you're just handed a pile of confetti you don't have that same correspondence.
If you're just handed a pile of confetti you don't have that same correspondence.
On the other hand physical shredders typically have other weaknesses, for example if you don't stir the shredded result you have a high correlation between original the current and original relative position of the individual pieces.