إِنَّ الْأَمَانَةَ لَعَظِيمَةٌ.
Surely trustworthiness is of great importance.
The presence of both inna and the emphatic lām is a secure grammatical observation. The claim that the speaker must be addressing a denier is only a possible contextual explanation, not a necessity of the wording.
A strong answer distinguishes established form—emphasis—from a possible function, such as rebutting denial or underscoring significance. This preserves precision while allowing legitimate contextual readings.