Can you really trust photocopiers with the microdots they encode nowadays?

If all you're worried about is redaction and not anonymity, a photocopier is perfectly fine for the job.

This about anonymity applies for whistleblowers which the forwarded article was about.

Aren't those only in color photocopiers/printers (relying on the lesser visibility of yellow ink)?

Not sure I would rely on that, but Xerox's docs state that the requirement was introduced as an anti-counterfeit measure by some governments. Counterfeiting currency probably wouldn't be a concern with B&W printers.

Most banknotes are not printed on ordinary paper, but it doesn't stop people from trying. Printers which produce those dots will also usually refuse to print high-resolution pictures of dollar bills.

> a permanent marker

Or better yet, an exacto blade. If the "black and white" photocopier is actually grayscale, then you could get text leaking through the marker if you play around with the image contrast.