I wouldn't necesalry worry to much about getting a bad review in a case like this. I'm unfortunately not superhost anymore, not due to reviews, but as I had to cancel a booking due to a mistake I made, had double booked a room (I had offered to pay up to double the price the guests had paid, for another place for them but they refused, so prefered to cancel so they would get their money back..).
But I still have over whats needed to be superhost, and come across this type of situation relitively often.
Sometimes I give refunds despite a strict cancelation policy if people are polite about it, give enough warning or if it was due to extraordinary circumstances.
If people leave a bad review for something like this (like if they ask if they will be refunded if they don't cancel, but don't cancel if I sa no so they can leave a review), if you leave a proffessional responce, explaining the situation and if your reviews are concistant I really would not worry about it... It may affect your rating a bit of course but that's where being concistant comes in, in the long run it doesn't affect you.