This is my opinion of this theory and it takes into consideration mainly the Fazbear Frights and Tales of the PizzaPlex books (tho I guess that could work with The Silver Eyes trilogy as well):
The idea of the books being in-universe fanfiction works so well for me, it's such a Fazbear Entertainment move to turn the real-life tragedies into creepypastas. If that ended up being genuinely the case I think they got written during the FNaF 3 era seeing the "Fazbear Frights" name and how both were made to make fun of the tragedy of the past and make people believe they were all just a "Slenderman" situation. There's also to say the in-universe VR game did the same and seeing how the books are continuing with "Tales of the PizzaPlex" I think they continued during the development of that game plus the PizzaPlex era to attract more customers and after the demolition of it to try gain enough money to open a new one.