if Kaido was strong enough at the time ... he won't say "try to become stronger"
don't misread that ...
Prime Whitebeard
Prime Garp
were still around
and they beat Kaido for sure
To be fair... there can be some mistranslation here.
But if it's true translation, you are right.
Although, THERE CAN BE some more deep meaning behind it than simply the idea of Kaido being so much weaker back then.
Like maybe Oden felt that Kaido has a big role to play in the future! And him being at the strongest form will help with that goal. So, even if Kaido was strong as fuck, Oden simply is telling him to go even beyond that.
Maybe it has big connection to the character who will show up in prophecy. So, the point I'm trying to make, there can be some deep shit beyond the scope of just "become stronger, you are weak now" LOL
Or maybe, the translation is off
But also, it can be just as simple as Oden telling Kaido that he needs to be stronger cause he's not powerful enough as you stated... who knows