Here "Jerusalem" is a metonym for the people who lived there. And, "all" is a generalization. It means all the most important people Alternate translation: "Nebuchadnezzar took all the important people away from Jerusalem" (See: [[rc://en/ta/man/translate/figs-metonymy]] and [[rc://en/ta/man/translate/figs-hyperbole]])
This can be translated as a positive statement. Alternate translation: "Only the poorest people in the land still lived there" (See: [[rc://en/ta/man/translate/figs-doublenegatives]])