Have you considered the wild possibility that he was lying to play the hype game/psych himself up, or that alternatively, he could simply be wrong?
GSP's movement, ability to change levels, cardio, etc...was way better towards the start of his second string of title fights than it was at the end and certainly better than when he came back at 185. That's obvious simply from the eye test, not to mention coming back from meaningful injuries.
It's not like the guy was lazy and ill-prepared back in the day. He was hyper-focused and prepared back then. He is now, except with more mileage, way more wear-and-tear, residual from injuries, and in the case of his last fight, extra weight that probably didn't suit him.
Well, good news I am a huge fan of GSP and his team, just not a blind nut hugger like most people, and I am obsessed about mma and sport performance, so I bookmarked a bunch of past interviews of him and his coaches about all kind of topics, including this one.
Firas Zahabi saying the same thing, listen to minute 4 and 30 seconds in this video
"The GSP of 2018 would best GSP of 2013".
Listen to GSP in these interviews, do you think he is lying for the sole reason to hype a fight?
Right at the beginning :"during the time off I had time to put my knowledge, all my techniques together.. I am much better.. in training I know I am much better version of what I was ..."
At minute 2 and 25 seconds :".. I don't want to come back just to come back, I want to come back if I am batter than I was, because the sport has improved as well ... the sport will catch up to you, you need to reinvent yourself.."
"I am not gonna be the old GSP, if I come back as the old GSP I am gonna have a very bad night the night of the fight, if I come back is because my trainers and my training partners are all convinced I am a better version than I was when I used to compete... I believe right now I have reached the perfect peak of athleticism, knowledge and wisdom as a fighter ..."
30 seconds in the video "... if I come back as the old GSP, GSP going to have his butt kicked, so I have to come back as a better GSP, with more creativity, I gonna have to come back with more creativity, better reaction, better tools and more opportunist.."
And minute 2 and 35 seconds " ... I have a few fights left in me, I don't want to do this until I reach the peak on the way down, now I am in my prime, I know it, my trainers knows it, training partners know it .."
Again, 30 seconds in this video: " .. I am at my best, in my prime, I would regret not doing this ... "
But if you listen to every GSP interview in the last 2-3 years he says the same things, he is in his prime right now, he is physically, technically and wisdom wise at his best, but he isn't going to be for long, so if the UFC can make the fights that interest him and elevate his legacy (Khabib) he is going to fight, if not, he is retired.