Understand that Jeremiah's preaching and writing took place after Josiah was dead, which indicates the repentance that took place during the reforms of Josiah was hollow. We will see this more clearly a little bit later. The repentance was just a surface thing, and not something that was really from the heart.
Here is an indication Jeremiah said this after Josiah was dead, because when Josiah was alive they were mending their ways. Josiah did what he did, because after reading the book of Deuteronomy, and especially Deuteronomy 28 that had the curses in it, he knew that they had just about had it.
