I remember him saying that. If I'm not mistaken, it was after his early UFC losses to Alvarez and Poirier. After that, he tightened up his style a bit and became more technical. He then quickly finished Barboza, Cerrone and another guy then put on a clinic vs. Tony.
Sadly, I think Max have been his career turning point just like it was Tony's when he lost to Justin. Maybe not as bad--Gaethje can still win fights, especially against other older guys. But in general he looked slower to me, and Max gave a blueprint for faster, more technical younger strikers to follow. And if grapplers can take Justin down, his submission defense is suspect.
Dariush-Gaethje might be a good fight to see where both guys are. I'm quite sure Justin takes it if both were in their primes, but if Dariush can bring it to the ground, all bets are off.