Thanks Zarax for answering my curiosity. 
Thank goodness you guys provide ALX.exe compatibility for your mod!
After you said it, now it's confirmed, because in XC using ALX.exe, I guess I've never saw the AI bring unretrained units in battlefield.(All this time I thought that's because some scripting in XC.)
Now I'm curious about other thing. 
What if I play 1.5 vanilla Rome but using ALX.exe, would the AI retrain their units and also would the AI do better in diplomacy and improve the AI's overall cleverness, even for just a tad bit? 
I asked this because this mate said so:
Code:
http://www.twcenter.net/forums/downloads.php?do=file&id=2739
I've tried it and honestly, from what I've experienced (I played as Julii against Gaul), the AI didn't retrain some of their units, but each faction acted more cleverly in terms of choosing allies and choosing their enemies.
BUT, this is just how I felt, I've not checked this thoroughly, this is just my assumption, but I'm pretty certain. 
I just need confirmations, because it boggled my mind for some time.
But the most important thing is, I've always felt the campaign and the AI in XC is much tougher and importantly, more fun than in vanilla. I'm very satisfied with XC.