Earlier quoted context omitted.
That stems from the fact that any definition of an insult will be either incomplete or incorrect.
Not saying you're wrong, but why is this a fact? Surely for a legal purpose you could make a pretty solid definition. If you want it to be fully black and white, you could enumerate all 'banned' insults.
It is a rhetorical flourish, but my gut says that language is complex enough to craft statements that both meet the legal definition but aren't insults, or are insulting but don't meet the legal definition.
I'm also discounting circular definitions like insults are statements that are insulting which are tautological, but don't reveal any extra clarity.
> you could enumerate all 'banned' insults
I'd argue that this is not possible due to the set of insults having at least countably infinite cardinality.