Guides from the 16-bit to PSX generations are pretty notorious for missing things. The official FFVII Guide missed a few items (Cursed Ring and Behemoth Horn) mixed up how to obtain certain materia (Final Attack and Sneak Attack), the FFVI guide for the SNES missed a few items and side quests, Xenogears guide leaves out rare item drops from enemies and doesn't realize that several supposedly unwinnable battles are winnable and net you rare items. Stuff like this are common for RPG guides.