Those successful "long battles" aren't really related with the recent AI changes. In the later stage of the game there is basically no difference in the strength "now and before" in all factions with exception of the magic faction.
The only difference which may be noticeable sometimes in later stage of game is a fact that now is quite much lower chance for stuck units. Taking into account that this problem was present only in some % of the battles, this is like random chance for the chance for no stuck units
, so slightly stronger AI.