The entire section for Undead Legion and those rules very clearly says these rules are for undead legion armies. It says nothing about errating the previous army books, it says the undead legion borrows units from those books and uses the army special rules in the Nagash book instead. The designer note clearly is referencing the absence of the general/heriophant dying from the undead legion army rules, same as the nehekharan undead designer note. So, if you want to use undead legion special rules, play undead legion. But the only part of the rules that modifies the army books is the morghasts, as they specifically state that they do. If slain general did, it would say ignore that rule in those army books, but it doesn't. It's saying that rule doesn't exist in undead legion.