Did they actually cancel the booking totally, so that the refund given was for 100%, or did they alter the booking to make the checkout date earlier? If they canceled, then it's considered that they didn't stay there and they can't write a review. If they stayed and just altered the booking to leave earlier, then they will have actually 'stayed' by Airbnb standards, and they'll automatically receive an email asking them to review the listing. You should also look in your dashboard to see if you have the option to review them. If you can review them, then they can review you.