If you cancel a guests booking then you get an automatic review to let other potential guests know. It's only fair as guests may have booked flights, made travel plans etc and then find their accommodation has been cancelled.
If you cancelled due to extenuating circumstances you should have contacted Airbnb to explain why and let them make alternative arrangements for your guests. Also, you would not have received this negative message.
I had been going to tell you to do something you already did - and that was to write a reply to the automatic posting. You did, and it is brief and clear and looks very good. Prospective guests know the reason for the cancellation and will not be put off. You still may want to take the other poster's advice to contact AirBnB, but in the meantime you have mitigated the problem.