A lot will depend on where your CC issuer (bank) is located. Does local law provide for chargebacks and other forms of disputes?

If so, you should pursue that immediately. Sixt will likely add you, perhaps permanently, to its do not rent (DNR) list and it may also sue you for the amount as well as its costs.

This boils down to a factual dispute as to whether you caused the damage and whether, you accepted responsibility (as the first email suggests). If you did accept responsibility, you have a problem on your hands.

For the benefit of others, when there is something off about a vehicle, do not accept it. I understand that there were logistical issues associated with this vehicle's location, but that does not change the underlying advice.