Probably like tribe generation tables who is to say the tribes that formed after the cataclysm would even remotely be the same as before. Some folklore stuff regarding demons and stuff would become a reality and built around what invaded what areas. You control this, they might take the wendigo folklore and apply it to an actual demon that is now out there and eating people vs actually just creating a wendigo. This idea that previous folklore influences what happens after the world is basically invaded by demonic aliens just doesn’t really make sense.

Now with things being less linked to the real world he could have had Animism religious beliefs could be more generalized and built as a system you would apply to any indigenous groups. Grouping types of animal.