"Did We not destroy the former peoples?" As an interrogative; meaning: Yes, We have indeed destroyed them; he means the past nations when they denied their messengers.
﴿ألم نهلك الْأَوَّلين﴾ عَلَى الِاسْتِفْهَامِ؛ أَيْ: بَلَى قَدْ أَهْلَكْنَاهُمْ؛ يَعْنِي: الْأُمَمَ السَّالِفَةَ حِينَ كذبُوا رسلهم