إِنَّ الْوَعْدَ صَادِقٌ.
The promise is indeed true.
إِنَّ الْوَعْدَ لَصَادِقٌ.
The promise is assuredly true.
The second wording combines “inna” with the emphatic lām attached to the predicate. The proposition remains the truth of the promise, but its assertion receives additional force. These are not two independently coordinated factual claims.
Greater emphasis may answer denial, but it can also serve solemn assurance, a pledge or a weighty announcement. Its appropriateness depends on the communicative situation. Consequently, the audience's precise state cannot be inferred merely from the presence of a single emphatic particle.