ירמיהו כב:יא פירוש: רש״י

concerning Shallum That is Zedekiah, and that is what he was called in I Chron. (3:14): “the fourth Shallum.” And he called him the Fourth, meaning the fourth to assume the kingship of Josiah’s sons, for first Jehoaha, reigned, then Jehoiakim, and after him Jehoiachin, and after him Zedekiah.

who left Even though he hadn’t left yet, he prophesied as if he had already left.