I don't know (I started playing with A15) but AFAIK those improvement attempts were probably tries to add new functionality to the old AI code, and those tries showed that it was unmaintainable and could not be improved to support bandits as well (from memory what Fataal said once). So between A16 and A17 the old code was thrown out and rewritten from ground up (by Fataal).
Probably it could have been possible to keep the old code and add a parallel code for bandits but that would have multiple disadvantages in my view:
1) Only one type of zombie intelligence, no way to gradually have more intelligent zombies
2) Double maintenance in case of changes
3) Impossible to add features to zombie behaviour
4) Complicated XML with different language for zombies and bandits
5) Some new feature they want could even be impossible to implement just because nobody wants to touch that old code.