It is voluntarily segregated. Non-black graduates CHOSE to not take part, because the description made it clear that this event was not for them. There does not need to be a policy or enforcement for segregation to occur. People self-segregate all the time. No one was explicitly prohibited, because that would be illegal, but it is clear that this celebration was being held for the black graduates. And the procession shows that it turned out to be exclusively non-white graduates that were the ones celebrated there.