Even prior to the patch, you would not be excommunicated for attacking a "cease hostilities against" faction IF the army was in your territory, seiging one of your cities or castles, or blockading your ports (naval). Those are defensive actions, and should not be prohibited. They were in MTW (sort of), but not now. Cool. You may also maintain a seige against that factions cities, or a port blockade. Just don't attempt another while sanctions are in effect. That, however, I think is a bit odd. I would still consider maintaining a seige or blockade as an offensive action. But I'll take it.