Taboos of Holocaust deniers
Revisionists have our own taboos. It is taboo to criticize the published writings or statements of revisionists who admire Hitler and the Hitlerian regime. It is taboo to publicly question the racialist arguments of specific revisionists. Taboo to argue publicly against the anti-semitism that exists among revisionists. It is not that we cannot do it, or occasionally do not do it, but we understand that when we do we will break the taboo against doing it, and we’ll suffer the consequences.
Of course, the reason it is taboo for Holocaust deniers to criticize their fellow Holocaust deniers for such beliefs is because virtually all Holocaust deniers are at least anti-Semites at heart, with some of them falling into the "Holocaust didn't happen but the Jews deserved it" brand of Holocaust denial. A not insignificant proportion of deniers do admire Hitler and the Nazi regime. And certainly some of them harbor white nationalist beliefs that drive their Holocaust denial. Granted, just harboring anti-Semitic beliefs that lead you to deny or downplay the Holocaust because you think "the Jews" are taking advantage of it to gain sympathy, suppress Palestinian aspirations, make money, etc., is a far cry from Nazi-ism or white nationalism, being of the same cloth of conspiratorial thinking everywhere, just with Jews as the source of the "conspiracy." But since Holocaust deniers are virtually all at least anti-Semites and consider themselves besieged on all sides, it is not surprising that they would be reluctant to criticize their fellow travelers for what are simply more extreme versions of what they themselves believe, much less to criticize them for anti-Semitic beliefs they share. In addition, the more clever Holocaust deniers try very hard to hide their anti-Semitism and insist they are "scholars" only interested in "correcting myths" about the Holocaust; they don't like their less discrete fellows openly voicing what underlies their own "revisionism" and undermining their attempts to hide the anti-Semitic core of Holocaust denial.
I will give Mr. Smith some small credit, though. At least he admits that there are anti-Semites, white nationalists, and Nazi sympathizers in the Holocaust denial movement. (He just doesn't consider himself one of them.) Most Holocaust deniers will try to claim that they are only interested in the "truth," that (according to them, at least) those nasty Jews (or, as Holocaust deniers prefer to call them, "Zionists") don't want you to know. To them, it's, "Nope, no anti-Semites or racists here." Smith has just openly admitted that there are indeed anti-Semites and racists there. He just can't admit that the whole enterprise of Holocaust denial is based on anti-Semitism.