That is a very good point. But I have looked at modern civil wars, and they don’t stay limited for long.
Tight ROE may be used at first, but as things esculate the State will start getting more and more bloody. If you look at Finland, Spain, the Balkans, and Greece (granted only the Balkan wars happened post WWII era), things get very bloody very quickly. Once everyone is forced to choose a side, the ROE will end.
That's entirely true, and to be fair, your point was that in CWII, there would be no ROE. That is largely correct. Once we get there, the game will change.
My point (and that of this article) was directed at the COIN aspects of the use of military force within CONUS. What wasn't explicitly mentioned, and should have been, is that this would likely be a transitional phase into a full scale civil war. You are exactly right in that the social mechanics behind the tight ROE would fall apart and the everything would likely go sideways. Still, how the initial uprising occurs, how it is responded to, and how the events are viewed by the public will set the fault line for how the earth breaks apart.
What the authors did was to sidestep this gigantic, looming concern by engineering an 'obvious racist villain' line in their story. No doubt they found this to be morally reassuring and uplifting way to write what is otherwise a disturbing scenario. Even if there was a 'break', it would be a full civil war but at 5% to 95% rate. The bad guys would be carefully distilled from the rest of society from the get go. Easy peasy. This is a comically simplistic view of counterinsurgency, where real insurgents blend in with and are supported by the people to an alarming degree. Reducing them to cackling cartoon villains is so colossally wrong-headed it brings great clarity as to how we wound up ten years into a dead end fight in Afghanistan.
Sadly, this inclination towards fantasy-based thinking prevents them from seeing the real danger of picking fights with strangers. They may wind up in a war when they merely wanted to deliver a beatdown. It also prevents them from considering that how the initial flare up starts and is handled will set the stage for the following conflict.