«زَعَمْتُمْ أَنَّ إِخْوَتَكُمْ قُرَيْشٌ»
You claimed that Quraysh were your brothers.
This claim prompts the question, “Was that true?” An explicit response might have been “You lied,” but the following contrast in commercial association and circumstances conveys the denial indirectly. The subsequent argument stands in for the omitted answer.