He probably understands the theory, but is not using the language correctly.
Purpose is defined as working towards something. Humanity is not working for something else, if it already contains all that it can be. You could say that our culture has a purpose, but even then, culture would have to be controlled in order to yield a specific result. Hell, even when you take science as a whole, there is no purpose as everyone is doing research or conducting experiments for different reasons. In the end, the individual decides the purposes (or lack thereof) of their own actions, and to speak of the 'purpose' of humanity, or the 'purpose' of culture is not very useful at all.
However, it does appear that human beings are adept at reproducing (hell, that may be the only thing we have a natural aptitude for) but that does not mean it is our purpose. Purpose is a human thing, and it is decided by each individual. Your purpose may be to reproduce, (lol) but that does not mean that every person's purpose is to reproduce. My purpose (chosen by myself) is to enjoy life, and to enjoy it in the company of others who enjoy it. Eventually, I would like to enjoy the company of my children. Eventually, I would like to enjoy the company of my grand children, but:
1. I do not have children simply for the sake of having children.
2. I do not have children simply for the sake of having grand children.
Do not begin to think that life has any higher purpose, especially not reproduction. Life just is. Enjoy it. Or don't. Nobody will choose for you, and if you decide to pursue another goal, nodoby will choose that goal for you.