The issue, as the article says, is that the law doesn't define what an insult is. That means that a death threat or saying "you're stupid" could potentially both be prosecuted under this law.
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.
It's irrelevant if the set of possible insults has infinite cardinality since we only care about the actual insults litigated on. If there is an insult which isn't in the list, the courts could decide if it fits their definition and through precedent it would be implicitly added to the list.
It's very relevant if you want a 'perfect' definition/insult list, but noone in policy land aims for perfection because they live in the real world.