There is a daily quest where it turns out your theory is right. I made a post about it around a month ago.

https://www.reddit.com/r/Genshin_Impact/comments/jprusc/little_nine_and_big_nine

Basically during the quest you are sent to find Chang the Ninth who is actually in the village where you find Little Nine. He talks about how he and the girl were neighbors and that she loved to follow him around. She asked him to write a book for her and so he was working on it (the book she took and drowned for). Her family didn't like him and so when she and her family suddenly disappeared, he thought they had moved away without saying anything because they wanted to separate them.

He devoted his whole life to making something of himself and becoming world famous so that the girl would head about him and they could be together again but disaster after disaster struck him and now he is almost penniless, writing stories for a novel competition in the hopes that it will become world famous and she will see it, all the while being unaware that she died long ago and that all the work he did to prove himself worthy of her was for nothing.

I hope he succeeds and that the little girl can read the book that he spent his whole life trying to write for her.