Intended as a joke or not is irrelevant. Making an exception for anti-Semitic and racist material is crazy and draconian? I don't see how. Seems like a perfectly reasonable and common restriction of free expression.
teaching a dog to raise its paw on command isn't really racist nor anti-Semitic, even in context. People heiled on the comedy Hogan's Heroes.
Nor is that why he was found guilty. Read the judgement if you are interested, it's fully and reasonably addressed.
If he wanted to use a free speech defence he could have done so at his trial, and he was specifically invited to do so by the judge. He chose not to.