It's a quick resolution for mainly off-screened fight.
Distraction and a cheap shot on the back of the head...
...and then came the Hour of Legends - showcase of Oden's endurance, a whole hour of boiling in oil, something even Kaido didnt deem possible so that's twice that Oden surpassed Kaido's expectations.
I don't think Oden was bullshitting, Santoryu was simply not invented back then yet and on the other hand, Santoryu is Oden's Nittoryu but with one sword more.
Bad writing should not prevent us from giving credit where credit is due.