What was Ash's reason to seek out Greninja to train with Lucario? AFAIK Lucario won vs Bea in his last match.Because with a major victory in the Battle Royal he'll think he can handle the Masters Eight. He'll only need training if he loses this fight and goes into a depression arc afterwards.
Storytelling isn't this binary or fixed. Ash could just want more preparation. Winning the Battle Royal doesn't automatically make him overconfident, especially against a bunch of people he beat already.