''The Grand God has made known to the king what will happen in...

''The Grand God has made known to the king what will happen in the future. The dream is true, and its interpretation is trustworthy.''

That is the claim being made. It cannot be shown that the narrative was not written after the described events.....or that this even happened, that the dream and the prophesy isn't a work of fiction, etc, etc. We only have the account according to the author. A common problem.