This implies that the city was breached on the ninth. A Mishnah (Ta'anis 4:1) says that it was breached on the 17th!
Ta'anis 28b, Yerushalmi Ta'anis 4:5 #2: The Mishnah discusses the breach before Churban Bayis Sheni. Our verse discusses Bayis Rishon.
Yerushalmi Ta'anis 4:5 #1: The verse is based on a mistaken calculation 1 .
R. Moshe Shapiro Ztz"l: Surely an event occurred on the ninth, and a newspaper reporter would have called this the day of the breach. Normally Chachamim, and all the more so Nevi'im, see deeper than this. Now they did not - "Ein Torah Gam Nevi'eha Lo Matz'u Chazon me'Hashem" (Eichah 2:9). Hashem allowed the verse to be written according to their mistake, i.e. a superficial understanding. Malbim (4) - the outer wall was breached on the ninth; Below (52:7) discusses a second breach - Tzidkiyah and all the soldiers fled. (Perhaps the Yerushalmi considers the latter to be the primary breach, and it was on the 17th! - PF)
What is the meaning of "Havke'ah ha'Ir"?
Radak (Melachim II, 25:4): They broke the wall of the city. When the soldiers saw this, they fled that night.
Malbim (2, 4): Due to hunger, the nation [opened the gate] and went out to Melech Bavel. When the soldiers saw that Sarei Bavel were in Sha'ar ha'Tavech, they [opened the gate] and fled at night. This was the second 'breach' (mentioned in Melachim II, 25:4 and below, 52:7).
