The point is not that every word changes its ordinary meaning under pressure. Rather, outward utterance and inner assent may diverge in a situation of severe compulsion.
If a person faces a life-threatening coercion to utter words contrary to faith, the condition of their heart is relevant to the legal discussion. This is precisely why the passage specifies مع اطمئنان القلب, an inward state of confidence in faith.
The example does not justify hasty judgments about other people's belief without establishing the circumstances and legal conditions.