«إِنَّمَا نَحْنُ مُصْلِحُونَ»
We are only putting things right.
This sentence presents the speakers’ own activity as reform. The ensuing Qur’anic reply overturns that claim:
«أَلَا إِنَّهُمْ هُمُ الْمُفْسِدُونَ»
Indeed, they are the ones causing corruption.
The first sentence claims reform for the speakers. The answer attributes corruption to them with emphatic expression and an independent pronoun. The pair shows why a rhetorical statement must often be read with its response: the latter reveals what precise assertion is being refuted.