What is Walter trying to say when he refers to African Americans as "the world's most backward race of people"? Does he seriously believe this?

Walter Jr. is depicted as a desperate dreamer who is fed up with his current occupation and desires to become a successful businessman. Walter Jr. dreams of using Mama's insurance money to invest in a liquor business, in an effort to eventually become financially stable. However, Mama and Beneatha do not support his dream, which pushes Walter Jr. to the edge.
During a conversation with Beneatha, Walter Jr. criticizes her for wanting to use some of Mama's insurance money to pay for her college tuition. Walter Jr. opposes Beneatha's dream of becoming a doctor and believes that she should become a nurse or marry a wealthy man. At the end of their argument, Beneatha praises Mama for opposing Walter Jr.'s dream and calls her brother a nut. As Walter Jr. is leaving the apartment, he looks at his sister and wife and says:

The world's most backward race of people, and that's a fact. (Hansberry, 41)

Given the tense situation and heated mood of the argument, one could argue that Walter Jr. made this comment out of frustration and anger. Walter Jr. is simply upset that none his family members will support his dream and his comments emphasize his negative mood. Therefore, Walter Jr. probably does not genuinely believe African Americans are the most backward race of people and was simply speaking out of frustration, anger, and disbelief.