He is not talking about the success of a few people in Harlem in the 1920's, until the 1950's most blacks lived under laws that set out to keep them separate from mainstream society. Segregation's goal was to keep blacks from assimilating with mainstream society. Blacks have not been welcome to assimilate with the rest of America until 50 years ago. Some still choose to live outside the mainstream, and they are ones who are causing all of the problems.
So what. Assimilation was no panacea(only a liberal would think that). Plenty of immigrants lived in unassimilated neighborhoods growing up and achieved success. Segregation does not equal failure. Thomas Sowell has written extensively on how much better African-Americans were doing scholastically prior to forced integration.