This implies that the city was breached on the ninth, like verse 3 said. 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 (Yirmeyah 39:4) - the outer wall was breached on the ninth; Yirmeyah 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 "va'Tibaka ha'Ir"?
Radak: They broke the wall of the city. When the soldiers saw this, they fled that night.
Malbim (Yirmeyah 39:2, 4): 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'. The first (mentioned in Yirmeyah 39:2) was that due to hunger, the nation [opened the gate] and went out to Melech Bavel.
What is the meaning of "v'Chol Anshei ha'Milchamah"?
Radak: All the soldiers fled that night. So it says in Yirmeyah (52:7).
What is "Derech Sha'ar Bein ha'Chomosayim", and why was it possible to flee through it?
Rashi: It is a cave (tunnel) that goes from [the king's] house to Arvos Yericho.
Radak: It is a gate between two walls. [The soldiers] fled covertly through it; the Kasdim did not sense this.
Malbim: There were three walls around Yerushalayim. The Kasdim breached the first, and the officers camped in Sha'ar ha'Tavech. The Jewish soldiers fled from the gate between the inner walls.
