إِنَّ الْوَعْدَ مَسْؤُولِيَّةٌ.
A promise is indeed a responsibility.
“Inna” marks emphasis, but its presence does not prove that the addressee previously denied the proposition. Emphasis may also highlight an important point, remove uncertainty or focus attention.
In the traditional distinction between initial, requested and denial-addressing reports, the hearer’s state is central. Reconstructing an entire psychological setting from one particle alone goes beyond the evidence.