I had to refer to that walkthrough as well since I couldn't remember, and it does say that the quest should be completeable right after beating the boss of the Evil Tower. It's been a while since I've played this game, and may be a while before I do again, if I ever do, so I can't recall for certain, but I think that I tried it right after saving Sarah, and the fairy still teleported me to the chest. That's why it's always important to leave back-up saves for games like this, since there's so much missable stuff.