What's considered personal information/doxxing?

I saw a screenshot of a conversation on a messaging app (between OP and another person) where OP hadn't blurred the name of the sender. Based on the name, another redditor was able to find their Facebook profile and reported in the comment section what they'd seen.

It's very much possible that the name of the sender isn't their real name, since it displays whatever OP had entered in their address book for that contact. It's also possible that the name the sender is using on Facebook is not their real name. However, they're the same or similar enough for another redditor to be able to find the sender outside of Reddit.

I reported the post as revealing personal information, but got a report back from Reddit admins saying no violation had occurred.

I'm not questioning the admin decision; I'm sure they looked into it and made the right call. I was just left wondering, what is the level of detail when it comes to personal information/doxxing? Is it considered doxxing if a random Reddit user is able to use the name to find the person outside of Reddit? Or are we strictly talking about legal names, addresses etc.?