This is fallacious and inappropriate logic. Exceptions to the rule do not disprove the rule. Furthermore, we know that there are will be people who will act against the best interest of their own group, sometimes sincerely, sometimes out of a selfish attempt to ingratiate themselves to the people in power. For example, we know that there African American slaveholders in the early history of the United States. We know that there were African American's who supported segregation and other oppressive systems in the civil rights era. We know that there were women who opposed women's right to vote.
Bottom line: Someone in an oppressed group denying the oppression does not mean that the oppression does not exist, and it is inappropriate for people outside of that group to use such as an excuse to deny real problems.