This appears to be a time-limited promotion.
> When we predict the price won’t decrease for select itineraries booked between August 13 and September 2, we’ll guarantee the price won’t drop, and we’ll refund you the difference if it does.
> When we predict the price won’t decrease for select itineraries booked between August 13 and September 2, we’ll guarantee the price won’t drop, and we’ll refund you the difference if it does.
If they have enough flight bookings per day, even a 1 day trial could be plenty to see how customers behave.