I'm not saying that the judgement is correct or incorrect, but he is being charged with libel -- a false publication that damages reputation. That is a crime here, too.
If he knowingly lied in a book and made false statements, and then people suffered damages (which has a lot of gray area to it) because of it, it's libel. ...that's the nature of the law here, as well as there. I think this is just taken a little extreme, because the "damages" are not very clear.
No he was convicted of Holocaust denial. If you openly disagree with any part of the uniformly accept number of killed, or how they were killed, you will be charged, tried, and convicted.
Currently there is some debate as to the number of actual Jews killed, as a nice round number of 6 million sounds a little to exact, even for the meticulious record keeping of the Nazi's.