The warning to flee Jerusalem is given to the people of Benjamin.
This may simply be a way of referring to the inhabitants of Jerusalem and its immediate environs because the city is on the border of the tribe of Benjamin and Judah.1 It may also be that some inhabitants from the tribe of Benjamin had fled southwards to find refuge in Jerusalem, but now not even that great city would provide safety from the invading force.2
1 Fuyez, enfants de Benjamin, du milieu de Jérusalem, Sonnez de la trompette à Tekoa, Elevez un signal à Beth-Hakkérem! Car on voit venir du septentrion le malheur Et un grand désastre.