Aim to be a Pokémon Master
It would be one thing to say that Ash being more even-tempered in XY was just a coincidence in the grand scheme of the XY plot. It's another thing entirely to say that Ash was intentionally rewritten in such a way so that it reflects off Serena better.
The directors basically just admitted that they couldn't write Amourshipping in with the Ash that was there in Kanto-Sinnoh, or the Ash in Unova. They had to actively write a different characterization for the boy, and add in (or in this context, fabricate) a flashback for the sole purpose of pushing the ship forward with no regard for characterization or context.
At that point, the simple fact is that the ship mattered more to the writers than the characters in the ship themselves. That's....de-legitimizing, from my perspective.
This is why continuity is such a big deal for people. Seeing how a character progresses emotionally can go a long way toward making a ship seem genuinely created, rather than shoe-horned.
Is that how he said it? One could also argue they have written Serena into seeing the more positive aspects of Ash, which were always there since his post-DP development. Ash has been here for nearly two decades and people to remember why they follow him in the first place. The majority of Ash's flaws have already been settled and tone down by the time he reached to XY. Even Sun and Moon didn't really belittle Ash unlike Best Wishes. In that show, he's more of a victim of circumstances and his classmates treat him with respect. So I say the it's the other way around. Ash's glory days was going to be XY with people coming to admire rather than look down upon him, and Serena's crush is just one way of showcasing that. Sun and Moon Ash would still smitten Serena as long as he is not rude to her or anyone else. And given how well he gets along with classmates, I don't think Serena will have that much problem with Sun and Moon Ash aside from him being the butt of the jokes (but I assume Serena is used to that during her adventures in XY).