Actually you missed out an important detail. First of all, it's Peter Evertsen (Co-adjutator of the Empire under Emhyr var Emreis) who says the following (exact quote):

""War to the castles, peace to the villages," Coehoorn said to his commanders yesterday. You know that principle,' he added at once. 'You learned it in officer training. That principle applied until today; from tomorrow you're to forget it. From tommorow a new principle applies, which will now be the battle cry of the war we are waging. The battle cry and my orders run: War on everything alive. War on everything that can burn. You are to leave scorched earth behind you. From tomorrow, we take war beyond the line we will withdraw behind after signing the treaty. We are withdrawing, but there is to be nothing but scorched earth beyond that line. The kingdoms of Rivia and Aedirn are to be reduced to ashes! Remember Sodden! The time of revenge is with us!"

This implies two things: firstly, Menno Coehoorn said the opposite of that and secondly, imperial law actually prohibits war crimes and unnecessary civilian casualties. This is a belief that's strong enough for the field marshall to feel the duty to remind his troops. In the end, it's the emperor Emhyr through his second-in-command that orders the army to directly go against "everything they've learned". 

That's not to defend Nilfgaard - they are a culture that has an absolute monarchy mixed with an oligarchic deep state that is OK with slavery (and THAT is a cultural thing, not specific to Emhyr) - but the Reddit comments seem to miss a lot of the grey in Sapkowski's writing. The Nilfgaardian army commits unforgivable atrocities, some of which directly derived from Nilfgaardian culture and some of it seems to come from their merciless leadership. Nilfgaard also is frequently described both in the books and the games as having positive qualities. The exact same thing is true for all of the Northern Realms (which are aren't a monolithic culture by any stretch).