Correct, and in most cases you would have to refund the ticket by 31AUG2021 (or perhaps 30AUG2021, not entirely sure and I wouldn't take the risk), meaning you have no choice to further extend the mileage expiry directly. You can, however, after getting the miles back, book another trip beyond 01SEP2021 (and within 353 or 360 days from the day of booking), either for a trip you genuinely plan to take, or hope the flight will be cancelled again, so you can refund again and extend the mileage expiry to 31AUG2022 then.