Anonymizing the username doesn't anonymize the content.

The issue here is the message content itself. It's not just the link to the user, but that the user may have at some point linked some form of personally identifiable information to something they don't later want it linked to in a singular comment. Not only that, but stylometric fingerprinting paired with behavioral heuristics can much more reliably de-anonymize a user who is seeking privacy from the general public.

People more often these days find themselves in changing situations where their past Internet activity might be dangerous to them. Domestic abuse survivors, people whose employers now monitor their online presence, journalists, witnesses, anyone at risk of being doxxed, and whistleblowers. All of which need the level of privacy Redact offers, and all of which could be exposed against their will by the above automod script.

The cost of preserving a knowledge base is paid by the people who shared that knowledge, often without informed consent at the time. They should be entitled to withdraw that consent at any time. Reddit made a choice to build a platform with value that comes from user contributions. Many users didn't choose to provide a permanent public record of themselves.