It's probably not private messages - but it is messages that people think should be private TODAY. But it doesn't really matter - it's perception that matters here. And also I guess this is an illustration of how people's attitudes to FB have changed over time.

"I would NEVER have shared this publicly." they say. But either they did, and now don't trust FB in the way they used to. Or they didn't and there's a bug. Neither are good for FB's brand.

There is no good way to answer "how do we correct a story that was completely wrong," but this seems to be a decent enough way to do it.