I feel KH made them more confidant in their ability to produce action games, but I would argue that the shift to Action RPG combat was likely more of them just following trends. Between all of the western style games that utilize it and the fact that some of the better Japanese RPGs of last gen were action combat based is the greater culprit here.

Hiroyuki Ito, who made the ATB stem in the first place, stated back in the PS2 era that he strongly believed the genre would move towards Action RPG combat as just a natural evolution.