It wasn't clear when elf stones came out that it was post apocalyptic.
Prequels eventually established that, linking that series with his Word and Void series.
I thought they made it pretty clear in the first book that it was post apocalyptic, it talks about the great wars where the humans almost destroyed themselves and changed the face of the world. It has the scene with the groups first big battle in the valley where they find the steel frames left of what seems like a modern era city.
As far as the series, I'm going to keep watching, but the writing an acting are pretty cringe worthy. Production values are really good, surprised they don't have better actors/script.