Judging from the comments on Archive of Our Own, many readers were hit by this that's completely unexpected by the author herself. Specifically, the readers follows Parasite_Ib's train of thought as the latter played the Sincere mod, finding the story behind "Saru-chan" (i.e., the "Monkey Girl"), and Parasite_Ib's anguish over the entire set-up. However, eventually they realized the Katarina in the Sincere mod is the exact same "Monkey Girl" Katarina from the canon.note It is implied that Acchan wrote "Monkey Girl" personality into Katarina due to her grief. This meant, they were reading how it would play out if Katarina's assumption about the universe—that the rest of of her acquaintances were just a bunch of code and she would eventually meet her doom.
is all about, switching to Katarina as she deals with this.