@Lee-Andre0 The customer service rep you talked to was probably going by the script that if you didn't cancel, it still looks like an active reservation for which you wouldn't be eligible for a refund.
You say it was too late to cancel, I'm not sure why you thought that, but when you talk to them again, tell them that- you would have cancelled it right away, but you didn't understand how.
If a booking doesn't get cancelled, Airbnb doesn't have any way of knowing if a guest really checked in or not. But in this case you have messages with the host to prove what happened, so they need to look at those.
Hopefully you get a more helpful rep when you contact them again.