It is worrying, @Donald28, with the phrasing "even though the trip was cancelled". That means it wasn't in error that the review is possible. Someone else mentioned this recently, too. You are not the first to see that message. Kind of scary.
I wonder what the criteria is? I have had a number of people cancel because of changes in plans, grandma is ill, etc., in part, I think, because cancelling is now penalty-free, and there have been no review prompts. Is it only when a refund is not possible? So only when the guest will be good and steamed?
The Help pages still say that a review has to be about a 'stay' (so one would assume 'not a non-stay'), and that it has to be the author's personal experience.
Are you up to phoning or tweeting to politely but firmly argue it out? Will you let us know how it goes?