unless you damage the car in a way which is not covered by LDW

anyway, Sixt appearantly applies the "german" way for deposits. if this is not accepted in US, they will learn and probably change it.
However, the branch in Miami is almost open for a year, if I remember correctly, and if this has not been an issue since, I doubt they will change something.

btw, for prepaid rentals the deposit will be much lower and is a fixed amount. probably something around 100-200 $