It makes sense for her abusive parents and toxic fiancee to be against her writing a book. We dont know the exact contents, but if it were about being a saintess, then just look at *how* she became who she is today. If it detailed any of her harsh training, it would likely be viewed in a poor light by the aristocracy/her parents.
Also, we know her home country is misogynistic, at least within the nobility (from the glimpse of a meeting in the past.) She is a woman, she is disliked by her family and betrothed and the nobility for being both extremely capable and not bubbly+beautiful+openly friendly like her younger sister. She is powerful and if given the chance, greatly independent. Of course they have an innate hatred for her.
What now, they were even against her writing a book? What's wrong with everyone in Gritonia? Is there a brainwashing spell? It's too much