Well he actually is not lying. Although enslaved through the use of spirituals and other coping mechanisms slaves held on to their dignity while being treated as less than humans and also the constitution does not guarantee dignity. Such language is not in the Constitution and Clarence Thomas's legal philosophy is that of an originalist -- therefore if it is not directly stated in the Constitution or its subsequent amendments such a right does not exist.
As an African-American, I appreciate that we do not all have to have the same group think mentality and are allowed to think for our own selves according to our experiences and beliefs (no matter how others may perceive those views) like every other race is able to.