Water starter fan
I can't rationalize it at all; it can't have been an oversight because they removed AG from two different frames that it should've been in, and even the possibility of a retcon becomes confusing when there was a very clear allusion to it with Ash describing it to GohNow that we know that AG won’t reappear… what was the point of excluding that form from that flashback? This is not something like the GS Ball that can easily be retconned, AG was, together with Team Flare, the major plot point in XYZ. They can’t erase all of this.
And for the sake of argument, let’s assume they did: What would the implications be? Did Ash seriously blast his way through Wulfric and the entire Kalos league and nearly defeated Alain with just a base Greninja? Ridiculous, really…