If NK cancels the flight or it is substantially delayed, you will be entitled to a full refund to your original form of payment. You have not provided a date of travel, but I would do nothing until then as most carriers are radically adjusting their frequencies and your flight may very well end up being one of them.

If, however, your flight is not cancelled, it will be down to your travel insurance as the offer of a credit / voucher is purely voluntary on NK's part. Don't forget that there are many people for whom a future date is useless, having nothing to do with their citizenship.

If you do want to take a run at NK, you must call. But, do not expect anything as I would not expect NK agents to have any authority to vary from policy.