I can't provide actual proof as CA's always been stupidly secretive about it, but i'm pretty sure that goes only for RTW engine where on harder difficulties AI units get substantial bonuses in attack/defense. In M2TW afaik AI gets bonuses for morale and stamina but not for actual combat stats, it'd absolutely feel if AI's units had such bonuses especially in situations like castle walls. Lot of big battles have just been walls of heavy infantry slowly wearing each other down and my (limited) cavalry numbers had hard time routing them if i didn't kill their general, but i never felt combat mismatches like that, something i absolutely felt in RTW when i played on VH there.