From what I understand, the Bishop does not deny the holocaust per se, but holds two positions contrary to the historically orthodox position (1) that the six million figure normally used for Jewish deaths is unduly high (I have seen one post stating that he holds that this is off by a full order of magnitude) and (2) that gas chambers were not the method used for execution, at least for the Jews. However, short of seeing the unedited footage of the original interview, the only way Bishop Williamson has to demonstrate his views are more moderate than they are portrayed is to speak again on the issue—but if they deviate at all from historical orthodoxy, they will be misconstrued in the same way.
No serious follower of Archbishop Lefebvre would deny the existence of the death camps, as the Archbishop's own father, a strong monarchist, died in one in 1944 after having been sent there for his work for the French resistance. (Discoverable by reading the first paragraph after the two paragraph summary of the Archbishop's bio on wikipedia). Of course, while this is a somewhat relevant fact, I think I've seen it pointed out once in perhaps 500 posts I've seen on the topic. That Bishop Williamson believes that at least hundreds of thousands of Jews died in concentration camps is something that most posters on various forums (and reporters) seem to miss.