The U.N. officials are treating this matter not as a thoroughly illegal invasion and military occupation of the sovereign nation of Syria, but instead as what will be a “humanitarian catastrophe” if Syria and its allies attack the area of Syria whose current residents are over 90% jihadists and supporters of jihadists. Syria and its allies are to be blamed for invading that jihadist cauldron, while U.S. and its allies are to be held immune from prosecution for their having used those jihadists, during the past 7 years — used them to invade and occupy not only Idlib but other parts of Syria.
They are not "moderates" in the Western sense. A moderate jihadi rubbing elbows with al Qaeda and IS is way, way, way to the monstrous end of the spectrum than are US moderates as presented in the US-corporate-mainstream media. The "moderates" are no democrats who desire the US Bill of Rights for the citizens of their countries. They are Muslims who view Islam as a total political ideology that must result in theocracy under Islamic clerics.
Do you want sharia law over Assad's secular ideology? I sure don't.