In fact, HV 4 says, "No member of the faithful could possibly deny that the Church is competent in her magisterium to interpret the natural moral law." I don't see how that can be interpreted as anything but, "we say, on nothing more than our own authority, what the natural law is". Which is a major reason why I reject HV -- when the supporting argument is nothing more than "this is true because we say it's true", the argument is meaningless.