John Calvin on Jeremiah 37:9

The Prophet confirms the former verse, and it was indeed necessary that this should be added, for though Zedekiah might not have divested himself of all anxiety and fear, he must yet have been moved by that prophecy, and thus he might have become more hardened in his obduracy, as it is the case with hypocrites; who, when they find that they can gain nothing, become furious against God, and run on headlong in their course. This might then have been the case with Zedekiah and also the Jews; hence Jeremiah adds, by way of confirmation, Elate not your hearts, or, Deceive not yourselves; that is, on account of the report respecting the Egyptian army. Thus he told the Jews that they had no reason to expect any alleviation. And the reason is added, For if, he says, ye had smitten, the Chaldeans so that few remained, yet they would rise up every one from his tent, and burn this city