The Kuribu in the trailer appeared to be made at least partially of stone based on the appearance of the boot, so I interpreted that as they are simply made of a stone-like substance and were mistaken for golems by later generations since Amdapor already had golems and there was no reason to believe they could be anything else. Alternatively they could have been intentionally "disguised" as golems by the Amdapor to keep the secret of their power very hush-hush. I don't think we can necessarily just accept everything we know as gospel anymore since already a significant amount of the "rules" we've been given by the lore have been bent.
Since the flood seems kinda analogous to the Rapture, and Sin Eaters analogous to angels, their purpose could very well be to spread and maintain Light. I keep on mentioning this specific one, but the Kuribu from the trailer did descend to the Derplander silhouetted by godrays, and after it was defeated night was returned to a portion of the sky. It could very well be they come into existence to quicken the Flood of Light before returning to the Lifestream. They may even be closer to Ascians than to Voidsent, at least in terms of their role and goals.
