I like OPs points, and think it's a solid theory.
But I've a hunch this is post SS, maybe 100 hundred years or so after; I think this will essentially about the formation of Hyrule.
I can't point to why I think this, it's just a hunch really, although it looks like the castle is being constructed which would make it early, and the lack of civilisation makes me think it could be during the colonisation of the world below.
But we'll see :D







