Jamieson, Fausset and Brown on Ezekiel 21:23

23. Unto the Jews, though credulous of divinations when in their favor, Nebuchadnezzar's divination "shall be (seen) as false." This gives the reason which makes the Jews fancy themselves safe from the Chaldeans, namely, that they "have sworn" to the latter "oaths" of allegiance, forgetting that they had violated them (Eze 17:13, 15, 16, 18).

but he, &c.--Nebuchadnezzar will remember in consulting his idols that he swore to Zedekiah by them, but that Zedekiah broke the league [Grotius]. Rather, God will remember against them (Re 16:19) their violating their oath sworn by the true God, whereas Nebuchadnezzar kept his oath sworn by a false god; Eze 21:24 confirms this.