Holocaust denial consists of claims that the genocide of Jews during World War II—usually referred to as the Holocaust—did not occur at all, or that it did not happen in the manner or to the extent historically recognized. "
Well, then you are right, of course, I didn't know this definition.
For me it looks like censorship of science, though, staunch defence of a dogmatic canon. Consequently, Irving looks like Giordano Bruno.
Anyway, even if he claimed that no single Jew was killed during the WW2, then stupidity is not considered a crime, in general. Only in this very case - and the law is applied to people who actually have something to back up their claims.
edit: the discussion has livened up... as usual