Israel was God's chosen people. He had saved them time after time, again and again, only to see them betraying Him once more. So they put away the foreign gods from among them and served the Lord . They confessed they had sinned. The confession of the sin is not necessarily repentance for the sin.
Genuine repentance has to do with the heart and it is accompanied by a change of the heart.
In verse 10, the Israelites confessed they had sinned but the foreign gods were still there. That's why I believe God pointed them to the idols when He said go and cry to the gods you have chosen . It is only in verse 16 that Israel put them away and started serving the Lord again. As soon as they repented, the Lord could no longer endure their misery. Judges 11 - 12 describe us how He delivered them from the people of Ammon.