Note: From the time since I originally posted this link, DuckDuckGo has clarified the status of search filters, and the article has been revised to reflect that. Thus, I’ve edited the Wayback Machine link to show the revised article. I apologize for any confusion, and I hope this clears things up!
Thank you for the link to the GitHub issue! I’ve found that the article I’ve linked has also been updated with mention of the issues with the search filters.