I don't think we are given any reason to distrust Kapugen's reasoning to his daughter. From what we see of their relationship, and the obvious pleasure that Miyax takes in her life and in the close relationship she enjoys with her father, there is certainly no reason that Kapugen would have to lie to his daughter. It is perhaps best to trust what he says to Miyax. Perhaps Aunt Martha had to remind him of the legal obligations that he has as a father to ensure that his daughter receives an education and a schooling. What he says about having to go and fight in a war is something that we have no proof of either way. Perhaps we could interpret his subsequent death after Julie is gone as a sign of grief at having lost his daughter, but we are given no indication as to whether Kapugen really has to fight in a war or not.
The main point of this section of the novel, which occurs in the second part, is to emphasise the massive change that Julie experienced in this period of her life. Note only does she have to leave her comfortable and stable environment where she is able to live close to nature and with her father, but she also loses that key relationship with her father and is taken to a radically different place, where she loses significant elements of her identity, such as her name.