Why would there be grey in a thresholded image? The entire point of the transform is that it maps everything above a certain threshold to pure white and everything else to pure black.
They didn't say "convert to greyscale".
They didn't say "convert to greyscale".
Very good point. But even then, assume that one page of a leaked document contains a large picture with areas around the thrshold value: With the agency being able to recreate a perfect replica of the initially scanned paper version, but without yellow dots, it might be possible to extract the (very few) bits necessary to boil it down to a single printer serial number by statistical methods.
...and they focus on adding fonts of multiple sizes so it can't be shrunk without losing information.