i bet Oda said that in a figurative sense,since he mentions kuina's vows right after.
i mean if you look at the plot now.. his captain has been fighting the strongest 1v1
and luffy got him worked up.
His goal here as a swordsman was to cut the strongest... but he isn't fighting the strongest 1v1.. and even ZKK i doubt would be 1v1.
so as it stands this plot itself is symbolic.
now can zoro fight kaido one v one in hoped to getting him worked up? ... i really doubt that... considering the drawbacks his sword brings and its not mastered yet and yea lets not forget Acoc mastery which he just realized he has.
so till EoS i'd expect luffy be the strongest of his generation, and zoro top be top 2 or 3