If Gura Gura is just a tool for WB, and Roger doesn't have an even more advanced form of Haoshoku, then they're just hype anyway.
Dude, Kaido basically just said this chapter (spoiler) that haki >> everything else, and that Roger became the top dog with just haki.
Zoro has advanced conq as well, does that mean his advanced conq = Luffy's? Of course not.
There are different levels to haki, that's really all that's really needed to justify the gap between Roger and Kaido.
They shouldn't be more powerful than the current Yonko. The Yonko since Roger's time don't challenge each other, the great Pirates' time was not their time.
So, is there a "PK level" according to you or not?
Anyway, this is just false based on portrayal and feats that we know of.
- Roger was said to be the king of his era. And WB was considered the defacto king of the seas by pretty much everyone, after Roger died.
There is a reason these two are put on a pedestal even by BM and Kaido.
- Old WB in meds had haki on par with Shanks, arguably the strongest haki user alive currently. This old WB, was considered to be the strongest pirate by pretty much everyone pretimeskip.
Prime WB >> old WB.
- Oden, WB's no2 and arguably peak Rayleigh tier by portrayal, is hyped to be able to give Kaido an extreme diff.
Oden was blown away by how strong Roger and WB were. And this was after he grew in strength, travelling with WB pirates for 5 years (so, no excuse of Oden grew stronger - because that had already happened, and after less than an year since then, Oden was back to Wano and faced stagnation).
If we go by portrayal, the gap between Roger and Rayleigh, or WB and Oden, would be the gap between Luffy and Zoro. So, whatever relative gap you think there is between Luffy and Zoro, should be the gap between Roger/WB and current Yonkos (considering Oden and peak Rayleigh are up there)
WB and Roger didn't challenge Yonkos merely due to the fact that Roger and WB weren't interested.
Roger and WB didn't feel the need to attack BM, Kaido etc, because they were pretty much irrelevant to their goals. Roger wanted to just explore and have fun, WB wanted to have a big "family". That's it.
"The Strongest Man in the World" can just be physical strength and end, after all Ace's novel points out that WB has the greatest physical strength among the Yonko.
Physical strength is just a general power level parameter.
The strongest in this context obviously means overall PL.
Or, are you gonna say Kaido is just the physically strongest creature? Of course not. This is just grasping at straws here man.
And lastly, we saw that old WB (Marineford, off meds) was strong enough to extreme diff Akainu.
When we see how strong Akainu is once we start flexing in the upcoming arcs, the whole idea of Kaido = peak WB/Roger is gonna swiftly disappear.