The story just says so. Not kidding, ylthenarrator straight up says "He's far stronger than when he left"
In the chapter where Oden starts dancing
The narrator says Oden came back far stronger than he left. He left like 6-8 years before that scene.
He met Roger like year before he returned to wano but a lot of shit can happen in a year especially considering the narrator straight up says he's been growing stronger
I'm aware of that scene. I still find the opinions of some ridiculous.
Oden was totally dominated by Roger 2 years before he came home. For the next 5 years he continued dancing, no fighting, no training, nothing. I won't buy that Oden doubles his previous strength in that single year it doesn't make sense. There was no reason for Oden to become massively stronger during the final voyage because there was no wall in front of the Roger pirates. WB let them by nor did the WG send any crazy force as we know. The Roger pirates were already the strongest with Oden, just his poneglyph skills were required.
Oden could already clash with Primebeard with like 27-29 ( don't remember how old he exactly was) and during their voyage they fought many battles as previously shown. He grew during that time, but not with Roger.
If the Oden that fought Kaido was a 100, i can't give the Oden that fought Roger anything below an 80, it doesn't make sense to assume that he was like half as strong as many assume.
Roger showed clear dominance against very near prime Oden. We know that characters can enter their primes during their early 30s