Malbim: Even though they properly repented, Hashem already decreed to exile Yehudah and destroy the city and Mikdash due to Menasheh. Mahari questioned this, for Hashem accepted Menasheh's Teshuvah, like it says in Divrei ha'Yamim II, 33:13, and Yoshiyah was a total Tzadik! I explained above (refer to 21:15:1:1) that the nation did not repent in his days. Also Menasheh repented from idolatry, but not from murder and other evils. Also, the nation repented in this year of Yoshiyah (when he found the Sefer Torah), but returned to sin in private - "v'Lo Yar'ah Bogedah Yehudah Achosah"; "mi'Kol Zenusah va'Techenaf Es ha'Aretz" (Yirmeyah 3:8-9). The 'flattery' was to appear good, but they were evil covertly and in their thoughts. The decree was sealed in the days of Menasheh; it was not torn via the current Teshuvah.