That is true. You don't have to explain it in the core text, and it might just be a waste of wordcount.
However, trying to say that it's inherently unexplainable, even by the Exalted -- that is an actual problem.
Absolutely, but I got the impression that Kaiya believes that it doesn't
need to be explained, rather than it being fundamentally wrong to explain it, and that it isn't so much that things shouldn't be inherently unexplainable as inherently...hmm, parsable, maybe? Which seem perfectly fine to me and within the scope of Exalted canon, because no matter how mighty or incredible an Exalt you are, you aren't going to be able to create an Exaltation, or really
understand what Lytek does when he's cleaning the Exaltations. Those two examples are canonical things that First Age Solars slammed their heads against trying to figure out, to no avail.
And if you
are going to go ahead and do those things anyway, then I'd say that doing so should be the major focus of your character arc, if not the entire campaign.
Exalted is a post apocalyptic setting as much as anything else.
This speaks against the idea that things should be irreprodicible and impossible to understand because in that case "lost knowledge" is no longer meaningful.
I want a Twilight scholar to discover fragments of treatises of those who came before and weep because she realizes that in the lost age their understanding of the universe was so much greater than hers, that she, a divine scholar, can only scrabble in the dirt and rediscover and apply a fraction of the understanding of that age.
I want the Realm to have shelves and shelves of books on magic that go untouched because in this fallen age the Shogunate equivalent of electrical engineering and computer science textbooks are no longer possible to use, the things they teach intended for a world that isn't one where water/fire duality engines are considered expensive and valuable artifacts rather than things made even by mortal hands.
I want a world where lost knowledge is meaningful because the world is explicable and the tragedy of the Usurpation is that this understanding of the world may never be recovered.
I don't want a world where all lost knowledge means is a couple of rarer-colored loot drops.
I cannot like this enough times.
However, to give my specific perspective on this, I want my scholars to uncover an ancient treatise on alchemy or the like, only to discover that half of it is utterly useless, because so many of the plant and animal species that it drew reagents from went extinct due to the Contagion. I'd like for them to search for a recipe for a rare and valuable potion, only to discover that its something that is actually commonplace in the modern day, because the rare species whose reagents acted as the bottleneck for production went through a population boom after most of its natural competitors died out. I'd like for ancient maps to be only starting points instead of guides because geography has changed so drastically.
I would like a world where knowledge is not only
lost, but gets
changed. Where sufficient context to loss is given that said loss matters beyond being collateral damage to the collapse of a golden age.