It was me he quoted. Faction Wars is more than just skirmishes when Bandits and Marauder, or Trolls and Imps, attack each other if they happen to be in the same area.
It's when one faction invades another faction's dungeon. I first noticed this when clearing out a fort while doing one of the quests for the Hoarfrost mod. The fort was jam packed with both Bandits and Marauder, even though only one faction was actually placed in the dungeon and I couldn't for the life of me understand where all the other guys came from. I think it was Corepc who explained it to me then.
I don't know the details, but there is a certain chance that a dungeon will see a Faction War when (I assume) the player enter. Not sure if it applies to only certain dungeons, either.
Ah, OK, if so I have never ever heard of or experienced that feature ingame. It's not mentioned in the MMM readme either as far as I can see. But Corepc would of course know. The closest thing I can think of is a feature in OOO rather, where Minotaurs and Ogres are set up to hate each other and also march in groups against each other. But I haven't seen that happen in ages.
I brought this up on the Team Alpha forum and it seems we're both partly right and partly wrong on this. MMM adds bandits to marauder lists and vice versa, which means you'll get Rockmilk in basically all bandit and marauder dungeons in a pure MMM game if I understood it correctly. This is toned down in FCOM - and more so with OOO in particular as it rules over a lot of dungeons. So in practise, it will be quite rare in an FCOM game - but I also have it in sections of dungeons and on outdoor spawns at times. Doesn't feel like faction wars in my game, perhaps partly because I use FCOM_SpawnRatesStronger.esp I guess, so I didn't think of those.
But thinking about it, I had a Rockmilk experience in a a couple of other dungeon ages ago, so I think it was when I played MMM + OOO really. Then it was a Rockmilk like war in every section and quite large at that.
Anyway, so it's MMM's jumbling up of bandit/marauder lists made rare by FCOM in general and OOO in particular.
EDIT: BTW, thanks for reminding me of Hoarfrost Castle. I've been meaning to check it out for ages, but I always forget about it as my chars don't use castles. But it seems awesome so I've finally installed it.