One long-standing element of both international law and just war theory is the inviolability of civilians or noncombatants.

This is true.

The just war theory had existed long before it was put into law. When it was put into law, it was done so with the Geneva conventions which had to be signed by countries and if countries did something against it, like kill civilians, they would get punished severely for war crimes and crimes against humanity. This is a key component of both practices.