Once upon a time, the secret of the Precursors was just that: a secret. No one knew what they were or where they came from. No one knew the full extent of their powers. No one knew why they all vanished. The only thing anyone knew was that they existed. Someone had to have left all those temples and artifacts and weird statues, right?
Many generations have studied the remnants of the Precursors' ancient civilization, but very little was known about them, even after hundreds, nay, thousands of years of research. The written word as it is known now is only a couple hundred years old. The most ancient of texts are still indecipherable, completely unrecognizable from today's script. What modern humans know about the Precursors has remained largely unchanged for millenia.
Then, one day, that changed. The Precursors were revealed for what they really were: ottsels. Small, furry, over-compensating-for-the-above ottsels. And it gets better: they weren't just little fuzzy rats. No, they were little fuzzy rats that could channel Eco as easily as the heart pumps blood. They were little fuzzy rats that built planets and ran the universe. One day, everything anyone knew about Precursors was turned on its head.
It's been three long years since then. Outside of a select group of people, the secret has stayed intact. After all, who would believe that their gods' true form was short and hairy and could easily pass as a common house pet? And what kind of chaos would it wreak if the truth were to become common knowledge?
So, yes, if you want to get technical, the secret of the Precursors is still a secret. But really, with a temple full of monks and a dozen others knowing the truth, and being personally acquainted with half a dozen of the Precursors themselves, how much more can there possibly be to discover about them and their history?
A lot, as it turns out.