Even private written communications can be libel if they are false and injure the reputation of the subject.

Not as part of a mass hack where one could just argue it’s fake data.

It isn’t that straight forward. If you wrote it and it got published, it still counts as published even if you didn’t publish it yourself. The crux of libel is that you made it permanent somehow by writing it.

Who's to say you wrote it and that the hackers didn't just insert that in the dataset?