This is an excellent example of fud created by security pundits wanting to say "actually, that's not secure". If you read that link carefully you will find that the problems discussed are not unique to digital redaction (size/length of the redaction, height of the redaction, etc). The accepted answer also clearly stated that if you use pixel-replacement (this is what DoD calls it) you're fine. Further, there have been plenty of times I've seen a redaction-by-marker break down when the underlying ink bleeds through in photocopy.

NSA published guidelines in 2005 for how to safely redact PDF (as well as word) documents. The corresponding physical redaction guidance begins by suggesting that the inspector physically remove the information to be redacted with an xacto knife. They redact and release thousands of documents, if nobody has reversed their redaction process yet then it's good enough.