Signals, symbols, and their respective verb forms are often used interchangeably, but there is a fundamental difference. A signal is shorthand for another thing because it is, in nature, directly related to that thing: smoke signals fire because fire creates smoke; red signifies blood because blood is that color.
A symbol’s meaning, on the other hand, is basically the punchline of an elaborate historical in-joke, which is then repeated so much and so out of context that the link becomes ingrained in popular culture.
Take, for example, 666. Everyone knows it as the number of the Devil; but what exactly is the number of the Devil? “Neron Caesar,” another name for Nero, transliterates into numerical Hebrew as 666. Further tidbits supplied in Revelations about the Beast and its associated phenomena directly imply the Roman Empire as their main inspiration**.
However, Christian Fundamentalists, heavy metal bands, and manga artists alike have all made references to the triple six in complete seriousness; when they say 666, they mean the Prince of Darkness, not the Roman Emperor!
Why is it that we may watch a horror film involving the Antichrist and shiver, without irony, at the sight of a three-digit number? Why is it that people may get a snarky dig at the Romans tattooed on their triceps and then proceed to flash it like a stolen Ferrari? Because the number of the beast is a symbol. A good chunk of any culture lies in symbols; that is precisely why writing pure fantasy is frustratingly difficult.
It is nigh impossible to eradicate Earth’s culture from stories about other realms. A writer looking to do a spot of worldbuilding either, in order to add some drama, has to reference myths from their own made-up world (and hence go into the trouble of creating them) or reference myths from our own very much real one. It’s easy enough to do the latter in an alter-Earth setting; it’s just this planet, after all, only magickified, so why not have another layer of meaning by throwing in old Celtic traditions? After all, we’ve all felt the warm enlightenment that accompanies the revelation of a funny-sounding name as a callback to something that actually existed in history.
Writers of totally imaginary high fantasy are stuck: one can only stuff so much Greek allegory into a story about dragons before it stops looking like a gripping yarn and starts looking like a metaphor - or worse, like a parallel universe.
And that’s great if you want it to be a metaphor, if you want it to symbolize. But it’s not so great if you’d rather have people just enjoying the work as opposed to scouring it for hidden Shinto messages. Of course, there’s always the option of manufacturing your own cultures, myths, and belief systems to make the world standalone, but not only does that take a colossal amount of time and effort, there’s also the possibility that references to alien epics will not resonate with readers. And anyway, given the human nature of every author, it is unlikely for a fantastical written work to be totally devoid of Earth references.
Regardless, good examples exist for every spot on the spectrum. The Lord of the Rings draws heavily on the medieval era, while Earthsea chose to forgo our world as much it could, instead referencing its own richly colored history. The Chronicles of Narnia are, at their core, Christianity, but Discworld, while similarly metaphoric, is relentless satire.
So there is hope. An aspiring fantasy writer is not necessarily doomed to allegory, and if they are, it does not have to be a hellish fate - although, it will certainly be hard work.