The problem, in Josh's estimation, lies with people imposing what he calls "objective-prescriptive," moral beliefs on others, when doing so may be sub-optimal for them. If the essence of morality is goodness, then any sub-optimality may be immoral for the person imposed upon. His argument is about maximizing what is ideal for each person. Nothing matters but the maximal epitome of what's positive and ideal.
To be more precise, he calls his system inter subjective-predictive morality or ISPM for short. ISPM is about giving each person what is morally ideal for them based on what they want, based on fact, rather than offering generalizations about what is right for everyone or what everyone should do, even if it isn't evident. Provided they aren't hurting someone else, there is no self-evident justification from barring someone from their best end good or from living their most valuable life.