A plain statement such as «فِيهِمْ رِمَاحٌ» could have communicated that spears were present. The response «إِنَّ بَنِي عَمِّكَ فِيهِمْ رِمَاحٌ» adds emphatic force, making the statement answer Shaqīq’s posture.
ExpressionRhetorical effect
«فِيهِمْ رِمَاحٌ»A plain statement of possession
«إِنَّ بَنِي عَمِّكَ فِيهِمْ رِمَاحٌ»A forceful reminder of neglected strength
The issue is not just that inna is an emphatic particle, but why reinforcement is fitting here. Its suitability is explained by the other speaker’s demeanor.