Law has a well-recognized optative use: it expresses desire for an absent or unattained state. In such cases it may be misguided to insist on supplying a normal, independent conditional response after every instance.
«لَوْ كَانُوا مُسْلِمِينَ»
If only they had been Muslims.
Although kanu is a past-form verb, the point in its immediate context is regret and desire, not a historical statement of an actual condition. An identical particle can serve different rhetorical purposes.