I had this happen in the USA, renting from Payless. 6 weeks after returning a car (I had rented for one day) I received a damage demand for $6500!
I had paid for the rental using AMEX Platinum and I am enrolled in the Premium Rental Protection Plan - pay a $17.95 fee to AMEX each time you rent using the AMEX card and they stand as first insurer. I sent the demand letter to AMEX, who demanded documentary proof of the damage from Payless. I never responded to Payless at all, just left it to AMEX.
About 6 weeks later, I received notice from AMEX that the claim was closed and Payless had abandoned the demand.
So my thought is you should not respond to SIXT, simply pass it along to your insurer used for the rental. Let them fight it out.