> Post-Rocks Garp had to train on mountains and still struggle to overcome Chinjao in a all or nothing kind of clash(the one who'd have lost, would have lost his main weapon)
> Prime Garp had to tagteam with Prime Sengoku to defeat Shiki, in a battle Oda judged to be impactful enough that half Marineford was destroyed
And Shiki has/had lower portrayal than Kaido, who Kaido himself admitted Oden
could beat / defeat. So it's not that unrealistic