It’s the difference between saying “The ‘president’ displays many signs of NPD” (easily supported) and “The ‘president’ behaves this way because he’s mentally ill.” (which excuses his arseholish behaviour as something beyond his control and also stigmatises mentally ill people who don’t behave like jerks).
What’s being asked for are: specificity (e.g. “NPD” rather than a generic “mentally ill”); supporting evidence if asked (e.g. a reference citing the DSM or a diagnostic tool); and taking care not to present one’s armchair diagnoses as authoritative.
