I suppose the Austrians disagree because after the Anschluss they joined the Nazis, and because Hitler was originally Austrian. Nazism needed to be rooted out. But I'm not sure that charging a foreigner for expressing his opinions sixteen years ago--or to put it more bluntly, for telling historical lies sixteen years ago--is helpful.
Nobody likes this kind of thing, but I don't think it's the equivalent of shouting "fire" in a crowded theater.
As you clarified, it's not an opinion. Irving is spouting blatant lies. Mentally ill nut jobs spouting similar incoherent nonsense have been arrested and ultimately committed to psych wards after being swept off city streets.
I would tend to agree with you in the end, but this is pretty damned close to shouting "fire".
Understandably, it crosses the line in Austria and Germany.