in my understanding, this was only for flights until 31.August. When the departure is from September onwards and the flight was not changed/cancelled by the airline you cannot "just" request a voucher and benefit from the 15% bonus.
Seems like this has been updated?
If you have a ticket for a flight initially scheduled before 31 March 2021, you can request a refundable voucher or a refund according to the fare conditions of your ticket. Please note that this request must be made prior to the departure date of your initial flight.