That doesn't explain why other similar letters, written with similar PR resources, always fail to meaningfully apologize.
I assume in those cases that's because they're up against constraints about what they can say without incurring legal liability.
Well, usually the source of information isn't U.S. intelligence agencies via the NYT
Not being facetious, but it is very possible that the person/people in the PR team who wrote it were familiar with similar situations or had a higher emotional intelligence.