The issue here is whether courts in Austria have the right to constrain people's public statements to some standard of demonstrable truthfulness, or not.
There's no meaningful slippery slope here either; because the vast majority of cases don't involve anything questionably true, they clearly involve statements that have evidence or do not, and where there's doubt, the protections are generally fairly solid (this is in the news often enough too; witness how Musk's knowingly false and clearly negative "pedo guy" comment was still not ruled defamation).
What's worse is the fact that you don't need to win a defamation lawsuit to punish someone if you're much wealthier than the defendant, because too many jurisdictions let each side pay their own legal costs - such that those wanting to stifle dissent can often simply threaten legal action, even knowing full well they'd lose. The point doesn't need to be to win, the point can simply be to impose costs that are felt unevenly.
Which doesn't mean there aren't some stupid, backwards laws that have no place in today's society, like for example the blasphemy law which weirdly still exists. There will always be room for improvement and it's never wrong to question established rules.
Note that this does not apply to just any kind of insult as you say. It's for cases when it's considered harmful for one's public image, as usual with defamation. I've never heard of a simple "Idiot!" leading to any consequences. I don't know why it's "incredibly fascist" to win a defamation case when someone called you a fascist in the open. Though I do admit that it's hard to draw a clear line with these things, and there are arguments for both sides as long as it doesn't steer towards censorship.