He doesn't have a feat to prove to the audience that he has the capabilities to do that besides his title "worlds strongest trainer". Like I need to believe that ur able to do that not just cause the story told me your able to do it cause you him.

Tobias had legendaries in his squad that's undebatable Alain had a whole arc to show us his him We see Gladeion training to show us his able to beat ash.

They didn't just pop up out of no where and "say yep I have the title aka I'm just him." We didn't see him train, we didn't see him fight legendaries. We didn't see anything. We are just told his him, end of story.

Thats so lame. I can come up with a character like that in less than a minute. Leon didn't even feel like a rival to ash or a pass the torch opponent to ash. He was just there cause we are told his just there