emailed them and got the following reply:
I apologize for any misunderstanding regarding the Hertz Status Match Program. Per the program specifications, Hertz will match your tier status with National or Avis. Hertz is not currently offering a status match for Sixt. In order to upgrade your Gold Service status I would need documentation reflecting an Avis Preferred Plus/President or National Executive/Executive Elite status. I appreciate the opportunity of clarifying this matter.
on May 19, 2017