I disagree, that would have been true if Mega Lucario had beaten Mega Garchomp, I think Lucario is still below Greninja, especially since he needed the frog energy to win
I'm not cricket so I don't hate Lucario by the way, but let's remember that Lucario needed the support of Greninja to defeat Togekiss, and Sirfercth to defeat Base Garchomp
Dumb argument both ways.
1) There is no way to say Mega Garchomp is > Mega Lucario ( I would argue Mega Lucario being stronger makes more sense narratively considering the narrative was about Ash overcoming Cynthia who was doing everything in her power to win. Not once did it seem that Cynthia had a handicap which allowed Ash to come out on top this time).
Mega Lucario battled a Togekiss that had taken no damage against Pikachu and then went on to Dynamax (+ Took on all of the Dynamax moves)
Base Garchomp only took real damage from Sirfetch'd who was hit by the Stealth Rock and also fought and defeated Milotic Before.
2) Similarly, no real rationale to say Greninja > Lucario or vice versa. That said, if Lucario does defeat Charizard; I think we could argue this point as it wouldn't narratively make sense for Greninja to be stronger than the World's strongest Trainer owned Pokemon.