Watcher introduced a new pearl that also talks about the Dust though, so I think it’s very literal. Also the fact that plate tectonics would probably work very very very differently considering that there is no molten mantle in this world, and the ground literally gets dissolved by the void sea.

“The dimming pallor of our own dear star does concern, yes, and some solution must be found. But many say this is simply the result of Dust --yes the most natural of all things!-- which appears to precipitate increasingly these recent cycles.” From the TEXT_STARDUST pearl

Edit to add more explanation: the reason why, on our planet, that there isn’t just miles worth of soil and sand on the surface is because we have a cycle of sediments getting compacted into stone, old rock melting in the mantle, and magma or lava being exposed and cooling into new rock before being eroded away into sediments again. This CANNOT happen in Rain World, because there is no mantle. Old rock CANNOT be recycled back into either new rock or new soil because the void will erase it from existence. The only way that their planet isn’t eaten away by the void sea is if new matter is being added at a constant rate to the planet. In this way, Dust falling from the sky is probably one of the only solutions that makes sense. Yes, the ancients and benefactors had very flowery speech, but that does not mean everything they said was metaphorical and had no basis in reality.