I went back and checked...That makes sense to me. In this model, I think I was reading the reflection backwards: I thought the mirror-spring was a sealed path between demons on the moon and the earth, but if we read it this way then the mirror-spring is a path for demons in the hand to reach out to the moon.
https://mangadex.org/chapter/e6acaf53-ef63-4cee-a6ec-e0d02cbfeab5/5
Way back, the goddess reached down grabbed a (her sized) handful of land, formed it into the moon, and placed it in the sky to act as a reflector for the sun. That act left the impression of the goddesses hand on the land, and in the center (palm) of that impression the moonlit pool formed. And because the land was taken from there, a connection was formed between that land (especially the pool) and the moon.
Demons sought to taint the pool and then with that, taint the moon, and pull the moon down to earth. They successfully tainted the pool, temporarily possessed the priestess and were well on their way to pulling the moon back to earth before Ryu stopped them, and pretty much reversed everything.
Except... Ryuu took a lot of damage. And she dropped the sword. Oops. Losing the sword would be a pretty damn problem considering how hard they worked to get it forged.
PS: I should mention that I haven't reread recent chapters, so maybe I missed something.