The Short Version
Hart did argue that the existence of law and its moral merit are separate questions — this is directly confirmed by his own 1958 Harvard Law Review article, where he called conflating the two "a confusion." The word "entirely" slightly overstates his position: Hart denied a *necessary conceptual* connection between legal validity and morality, but acknowledged that law and morality are empirically intertwined "at a thousand points." The claim captures the substance of Hart's thesis accurately, with only minor overstatement in framing.