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According to some of the earliest Qur'an-commentators (e.g., Qatadah and 'Ikrimah, as quoted by Tabari), this threat of ultimate doom applies to evildoers of all times - which further supports the assumption that the expression hijarah min sijjil has a metaphorical connotation.
If we take the words literally, they would mean that the showers of brimstones were marked with the destiny of the wicked as decreed by Allah.
They: Arabic, hiya: some Commentators take the pronoun to refer to the wicked cities so destroyed: the meaning then would be: those wicked cities were not so different from other cities that do wrong, for they would all suffer similar punishment! Perhaps it would be better to refer "they" to the stones of punishment by a metonymy for "Punishment": 'punishment would not be far from any people that did wrong.'