Koro Apirana is angry at Nanny Flowers because he thinks that she chose Kahu’s name specifically to defy him.
In reality, Kahu’s mother Rehua chose the name, and she did so not to provoke Koro Apirana but rather to solidify her daughter’s link to him. It’s noteworthy that Nanny Flowers doesn’t tell Koro Apirana the truth. Perhaps she wants to protect Rehua or the people of her tribe; or perhaps she thinks that defending Kahu’s name is more important than identifying who chose it.