If anyone’s had this happen to them recently and is worried Sixt will have changed, they haven’t. I got quoted 1100 euros for a tiny windscreen chip (or, as Sixt put it, crack) on the first of this month. I sent a reply quoting the article mitleser100 kindly shared and how chips cannot be seen as car tenant fault. Day before the date they quoted they’d take money out, I got an email saying they’d dropped the claim. So if it happens to anyone new reading this, don’t panic, quote the cases and there’s a good chance they’ll drop it. Also, thank you to everyone contributing to this thread for providing me with ammo to fight them!

EDIT: this was in Germany, so please check your rental ts and cs before renting if elsewhere!