Just make sure to read the Terms and conditions before you get too excited.
The promise is only valid for 11 days Bergen-Kirkenes-Bergen voyage - you’ll get a new 6 or 7-day voyage in an inside cabin (includes all meals) but no additional travel (flights).
A new trip must be ordered within one month of returning from your original trip.
Also it’s very unlikely at a 11 days voyage not to see the lights at all. “The ship determines whether a Northern Lights event has occurred, regardless of weather conditions and actual visibility” - and time of the night.
The Havila and Hurtigruten ‘promises’ are very similar.