Understand you are annoyed, but Airbnb provides advice for guests who find their listing is not as described on your booking confirmation @R48
It advises you to contact the host within 24 hours of arrival and if the host cannot correct the situation ie provide you with the bed advertised, then you can contact airbnb to cancel your booking.
Did you this?
What you can't do is stay there and still ask for your money back.
It is not Airbnb's fault that the host has misled you, that is clearly the hosts fault.
Surely you don't expect Airbnb to send a rep to each of the millions of millions of listings on a daily basis to make sure everything is as describe do you? 🙂