Someone may truly believe inwardly but be unable to speak, prevented from speaking, or subjected to coercion. Inability to move the tongue is therefore not automatically evidence of inward denial.
The absence of an uttered profession can arise from very different circumstances: refusal, lack of opportunity, physical inability, or coercion. These cannot all be assigned the same judgment about inner assent.
Public legal status still follows reliable available evidence, which may include an appropriate sign or other established indicators. Complete knowledge of the person's inner state belongs to Allah.