This chapter has a lot of baki energy.
Kaido and yojiro are both called "the words's strongest creature", who's looking for a worthy opponent.
Kaido's treatment of yamato is very similar to the way yojiro was treating baki, so maybe kaido was grooming her to grow up into the worthy opponent he always desired, who can make him fight to his heart's content.
We know that kaido has lived as a prisoner and endured all kinds of torture, so when he witnessed yamato's CoC he realized her potential, and decided to force her throgh similar experiences to his, at the hope it will make her stronger.
It makes no sense why kaido would leave a bunch of swords for impisoned but not restrained master swordsmen in a stoney cave, it feels like a plan, also the handcuffs are the same that yamato was wearing for 20 years, and they're exploding cuffs, designed to prevent yamato from leaving onigashima, so kaido maybe wanted yamato to escape from the cave.