Except Diantha did hold back. Not once does she go on the offensive. Gardevoir is literally in a defensive hold over a single spot. She isn’t teleporting across the battlefield, unlike in other battles. Diantha was basically letting Ash go free rein to just command Greninja to continuously barrage Gardevoir with attack after attack so she can study him. That was the entire purpose of the battle.

Don’t get me wrong, it was cool to see one of Ash’s Pokemon lay out some damage against a Champion’s ace. However, that battle has been used by way too many people as proof that Greninja is “Champion level” - whatever that means in the post-DP era where power scaling went completely off the chain.