جَاءَ شَقِيقٌ عَارِضًا رُمْحَهُ إِنَّ بَنِي عَمِّكَ فِيهِمْ رِمَاحُ
Shaqīq came displaying his spear; surely your cousins have spears too.
The hearer has not verbally denied that others possess weapons. But the boastful presentation of a spear suggests disregard for their strength, which resembles denial.
The speaker therefore uses “إِنَّ” as though correcting a denier. The rhetorical clue is the boastful attitude signaled by the action, not merely the mention of a spear.