Fictional stories are a little different than just idle joke telling because they can be used for educational reasons. Fatawa saying they are permissible likely were referring to fictional stories that serve this purpose. (Even cartoons are allowed for young children for educational reasons, despite being 'images').
If the above does not suffice for you, then it's an Islamic principle to leave that "gray area" ie, doubtful matters. Prophet Muhammad (ﷺ) said about doubtful matters: "whoever leaves it to protect his religion and his honor, then he will be safe" - Tirmidhi (1205).