إِنَّمَا هُوَ إِبْلَاغٌ مَعَ التَّخْوِيفِ
It is communication accompanied by inducing fear.
Tahdīd makes the audience fear the consequences of misconduct. Indhār foregrounds communication of impending danger as an admonition; its semantic scope includes informing as well as arousing fear.
Both may occur in related contexts, but they differ in emphasis. A threat directs attention to impending adverse consequences; a warning overtly communicates danger. Neither should be flattened into ordinary permission simply because imperative morphology is present.
FeatureThreat (tahdīd)Warning (indhār)
FocusFear of consequencesNotification of danger
PurposeDeter defianceAlert and caution
Imperative forceNot genuine permissionNot just soliciting an act