Took me a bit but they're replacing the string characters which are numbers with '0' and keeping the rest. So "3 102 45" becomes "0 000 00". Once I got it I thought it was brilliant.
Also, the author could claim they inspected/analyzed the visual output and the redaction appeared flawless.
At least, I assume most image viewers won't display a dialog:
Warning: suspiciously redundant image data detected. If any part of this image is supposed to be redacted, please contact your system administrator immediately.