Yeah honestly the ending was bit of bummer. You might have noticed that Celceta starts by some random person narrating the story. So from what I found online about the ending was that apparently it's either the narrator who remembers the story about that specific Adol's Adventure only till that point or whatever book or some other piece of literature about Adol's Adventures he was referring to ends at that point.

But still compared to other Ys games, the ending was a serious let down. Probably the ending was rushed maybe due limited time or budget, but that's just a guess. But other than that it's still very enjoyable entry in the Ys franchise.