Dr. Poussaint is really referring only to extreme racism, where the person tries to harm or kill members of a particular group because of certain beliefs about that group that Poussaint considers delusional.
In most cases, there is both a cultural basis, as well as some cultural support, for these extremely prejudiced beliefs, even when they are irrational or inaccurate and based on distortions, and, in some instances, there is also cultural support for taking aggressive action against a particular group, so I would not agree that the thinking involved is delusional in nature and I would not see it as reflecting mental illness.
By Poussaint's standards, the members of the KKK would be considered mentally ill, particularly when these members regularly engaged in lynchings, the Nazi's would be considered mentally ill in their attempts to exterminate Jews. I think these types of examples indicate extreme anti-social behaviors and extreme prejudicial attitudes that unify particular groups by designating a common enemy that serves as an outlet for the group's aggression. These groups are extreme in their prejudices, and in their actions, but I wouldn't regard this as indicative of mental illness.
While Poussaint wants extreme racism considered as a mental illness, so that it can be addressed as a treatment issue, I think that it is better considered as extreme sociopathic behavior and regarded as criminal, and a hate crime, in terms of how society understands and addresses it. I agree with Poussaint that such individuals pose a danger to others, but not because they are dangerously mentally ill. They do not lack the capacity to control their actions, they do not lack the capacity to differentiate right from wrong, and, when they act in a way to harm members of a particular group for prejudicial reasons, they should be held fully accountable and responsible for their actions, and not just in need of treatment for alleged "delusions".
Fairly recently, there have been instances of Hispanic men being randomly selected and beaten to death by small groups of teens holding extreme prejudicial attitudes toward Latinos. Instances of gay-bashing are other examples of extreme prejudice, as are fire-bombings of synagogues, such as the very recent case where a rabbi and his family were asleep in the building when a fire of this type was started. And instances of hate crimes toward blacks still occur. In all these instances that I am aware of, the individuals involved were aware of the nature of their acts, no matter what rationalizations they used to justify these actions, and they did not lack the ability to control their impulses. There is a significant difference between rationalizing a prejudicial action and being compelled by a delusional belief which has no basis in reality and no societal support whatsoever.
I do not doubt that the individuals involved in these sorts of extreme prejudicial actions have personality problems, with likely paranoid features, that predispose them to acting-out and scapegoating in this manner, and that treatment for those problems should be considered a necessary part of rehabilitation for a criminal offense, but I do not regard their acts of hatred as indicative of a delusional mental illness. To suggest that it is mental illness implies we should withhold moral judgments of such behaviors, and I would not find that acceptable. We have every reason to view the harmful behaviors that derive from extreme prejudice in moral terms, and to consider such behaviors as abhorrent, unacceptable, and criminal. And the basis for these behaviors could best be understood in terms of an interaction of socio-cultural factors and individual personality dynamics. Just slapping a diagnostic label of "extreme racism" on such individuals would explain nothing, nor would it change how society must respond to their behaviors. It's just a needless diagnostic entity.