So a purpose, then, is what the organism does. Hedonism, I maintain, is what drives or motivates us to do what we do. The details obviously vary considerably from one situation to the next. So then their purpose is to live. No, I would argue that their lives have no real subjective purpose, since they are biologically incapable of having such a subjective meaning to their living.
Humans also live.
In fact, our hedonistic actions are contingent on us living. You could say that our living is also contingent on our hedonistic actions, but that isn't true. One can live and not pursue pleasure. Freud knew this. People starving in third world nations know this.