They say Oden ~ Current Kaido because Kaido couldn't have grow stronger in 20 years since he was already 39 in the flashback... which was Roger age in GV who was supposedly far from his prime all of sudden
Kaidou wasn't fighting in the meanwhile.
There are no recorded battles between that timeline.
Whereas Roger occasionally fought with Garp and Whitebeard. And even Shiki was there.
Two different set of comparison here.
Most people in one piece hit Peak at around 39/40.
Which happens to age of Blackbeard , shanks and mihawk all together.
Even Aramaki is 41 rn.