A skeptic would say yes, of course they are, but that's not what I mean. We have ancient texts in which people are described as making up false stories and false gods. I'm certain it happened. We just don't know to what degree.
I'll try to explain.
One could easily say that the Mahayana sutras are fan fiction of the Buddha: They use the Buddha as a character, but by all serious scholarly accounts they were composed many centuries after the Buddha lived, and in a very different culture. They are full of anachronisms and ideas quite foreign to the Pāli Canon, the earliest texts that Buddhism has. This doesn't necessarily mean that Mahayana sutras are devoid of a Buddhist message, or that their authors understood themselves to be engaged in a fraud. They were telling stories about truth, as they understood it, but with elements that they did not personally witness.
Even within the Pāli Canon, there are clearly multiple voices and intentions; there is even at least one pericope that I can think of that seems not to be the words of the Buddha or his immediate followers, but rather a set of later instructions on what a monk should do when encountering parts of the still-developing canon that are unfamiliar and possibly at odds with their personal understanding. This could only have been produced in an environment where text composition was happening fairly freely. The Buddha was probably an actual person, but he seems to have acquired legends just like Heracles did. (Euhemerism is the term for this particular phenomenon.)
The Canon itself--the fan fiction of the Buddha--was first built up against the background of a religion that has left us hardly any traces. The local Brahmanic religion of the Buddha's contemporaries had gods, rituals, symbols, and doctrines that are described in the Pāli Canon as constituting the religious life of the era. They bear only a vague resemblance to Hinduism today. (Perhaps because in Buddhist texts, the Buddha convincingly bested all the old gods?)
The process of gradual slippage from perhaps deliberate fan fiction to earnest religion also explains a lot. One could say that the Book of Mormon is Jesus fan fiction, and that the Gospels are messianic Jewish fan fiction. The Greek myths are in a more or less canonical form now, but they were also very clearly the product of a process of collective storytelling, with borrowed characters and later reconciliation. Levels of earnestness and literalism varied in pagan cultures just like in our own, as Cicero noted.
Now for some extreme cases. In the present day, some people profess that their religion is "Jedi." And they mean it. And the Jedi religion, such as it is, has a more or less respectable philosophy behind it. The same is true of the Church of All Worlds, based on Heinlein's Stranger in a Strange Land, and to a much lesser extent Scientology, whose principal doctrines are found allegorically in Battlefield Earth. Philip K. Dick spent his last productive years writing both fact and fiction about his religious experiences, and he later confessed that a lot of his earlier work seems to anticipate his later embrace of a sort of gnostic Christianity.
Fast forward ten thousand years to the future. Buddhism is still around. So are the Jedi, with their legendary ancient videos of Luke and Leia. Nobody knows how to lift objects with the Force, but we just accept such creative embellishments in an ancient and revered religious film like A New Hope. The Jedi religion is all about finding a balance in life. It's about suffering and the way that we avoid it. It's about a peculiar martial art too, and practitioners insist that it's somehow important to the process. Was Luke a contemporary of the Buddha? Certain shared ideas and terminology seem to say yes, but the more serious scholars are divided about whether Luke even existed at all.
There are several spectra along which the continuum between religion and fiction can be discerned: there's time, of course, which gives license to myth making and may make a conscious fiction into a myth. There's textuality versus lived practice, which obviously varies by time but also from person to person. And there's authorial intent, which can obviously vary--on a continuum--from "I am writing Star Wars fan fiction" to "I have just received a literal message from the Living God." But one can emphatically lie somewhere in between those two, as perhaps the Mahayana sutra authors did, with George Lucas and John Smith each on opposite poles.