טובים השנים יותר מן האחד. Thank you to Fred for tracking down a source for my complete guess at an answer.
R’ Chanoch Henoch Shafran of Bucharest, in his footnotes to his father’s responsa, Shu”t HaRavaz OC 1:27, asks a similar question on the Chida, who quotes it with the word דכתיב, which throughout Chazal invariably introduces a Passuk of Tanach, when הן גאלתי is not found in Tanach. He answers:
אולי כוונתו במלת ״דכתיב״ היינו: דכתיב ״בנוסח הקדושה״.
Perhaps his intent with the word “as it’s written” is that it’s written in the text of Kedushah
(as opposed to the text of Tanach).
Similarly, we can perhaps answer your question on the Kedushas Levi by saying that he doesn’t intend the word פסוק to mean a verse of Tanach, but rather a verse of Kedushah.