Still it s odd and yamato calls herself son of kaido . Honestly its bull writing just to throw surprise on readers.
Honestly its another plot that kiku is a dude but he likes being woman.
We'll need more info in the upcoming chapters to know why Yamato calls herself Kaido's son.
It could be that Kaido raised her as such from her birth and now she is so used to play that role that she actually considerable herself a guy even when she is not.
Or it could just be to throw the readers on a misleading curve last chapter to reveal a bomb shell this chapter.
Look at Yamato's build,compare it to Kaido and the calamities that are are of similar heights,you can clearly see Yamato is much skinnier than the calamities with nearly no muscles for a guy of that height who is nonetheless Kaido's son,Wears a mask and keeps long hair with no topknot(like the other samurai)despite wearing a Japanese type dress.
I definitely think there were some hints that Yamato was a girl.