The submitter here is Matt Might, whose family was the subject of the article. His first comment in the thread makes it obvious why this is an exceptional case.

With all due respect, you've only been on HN for a couple of months.

Why would there be an "exception" to the policy of discouraging submitters from editorializing headlines? Even if the submitter had a personal connection to the material (which isn't unusual), the guidelines should be respected.

Because the purpose of the guidelines is to make the site better, and changing that headline would not make the site better.

Also, the headline modifications in that case were not made with the same intentions that other (harmful) headline modifications are made.

The source article was discussed here: https://news.ycombinator.com/item?id=8035726