As I read that, users must trust you. There's nothing that would actually prevent you from accessing the data.
So it's arguably misleading to call it a "private diary".
But none of that is relevant for "private diary". There, it's privacy between you and your diary.
But that's the problem, isn't it? I mean, many people do (sort of magically) think that their online stuff is private. And then they get pwned in one way or another.