So there's no proof that kaido is yonko tier. Okay.
But I do have some proof he got stronger.
Oden tell him to get stronger. Remember. As he's boiling, oden tells kaido he should get stronger coz his samurai will come fuck his ass up if he doesn't.
And we know Kaido has some weird respect for Oden.
You can say maybe kaido decided not to follow oden's advice but then again there's also no proof that he specifically said he didn't follow it.
That statement exists and there's no proof Kaido was a yonko 20 years ago.
So there you have it.
bro, it doesn't work like that. If words were enough to make anybody stronger we wouldn't be there.
There is something called power level glass ceiling : Oncee you reach a top tier rank, it's near impossible if not totally to progress.
That's why luffy beating a yonko or two in this arc won't mean he'll be even EOS leagues above them.
Same of why yonkos are all more or less in the same tier power wise, they just reached a rank where it's impossible to progress unless plot most of the time, or the power sclaling and power balance in the world would have been fucked.
It's like when you reach nuclear power level in real life, after that there is nothing that you can really do.
And you have to admit that if anything, Oden was on par with prime raylegh, who is already above admiral tier, in prime since he stalled as an oldcel and with ease of one the strongest admirals.