The myth/ideal of being "fully human" is an example of essentialism, and is one the techniques that authors like Huxley and many from his era employed. It preys upon the inner authoritarian desire to normalize the world for oneself above all else, even though ironically Huxley opposed authoritarianism himself.
A different take is that people's mental health as individuals is in a constant tug of war with their own circumstances, which are then in their own tug of war with the collective stresses of society. There is no normal/abnormal spectrum as Huxley would have us think. Just a complicated web of complicated agents, sometimes in harmony with their society and other times not. When the net entropy to the agent is relatively low, that's "normal". It's not a value judgement or a mark of inauthenticity the way Huxley might have you think.