Grant

I dont think he mentioned The Flame of Udûn as "a nod to book fans who would of gotten that reference" as The Silmarillion wasn't published until 5 years after Tolkien had passed away (by his son combing through his notes about the world he built) and 23 years after Fellowship was released. Seems more likely that it was included to make the world feel more fleshed out with the characters references indicating the knowledge Gandalf has of the history and vaguely how he relates to it. The books didn't delve deep on things like Númenor or the Valar, but mentioning them did help tell the reader "there is more to the history of the world beyond what is mentioned" and one of the reasons Tolkiens work remains one of the best world building. Dude could just drop lore whenever he wanted since knew it all and didn't mind leaving a few loose ends (like the blue Istari) because real life is full of those too. We just got lucky a few things were answered with the release of the Silmarillion tbh