@Michael5689
Well, sport this happened to you, but it's down to some basic realities.
1) there is no "security deposit". It is an illusion. It simply doesn't exist. No money is "deposited" by the guest to cover any potential damages.
2) "Air cover" Is not a "guarantee" nor "insurance". This is one of the few terms that isn't vague. Ok, they paid out 20%. Count your blessings. They didn't have to pay anything.
It's always better to try to work it out with the guest and keep Airbnb out of it. In this case, the guest admitted it, and perhaps would have paid for the full recovery cost, by providing the receipts.
We've had to absorb many costs like this. It's just not worth pursuing most of the time. If they started the bedroom on fire, that would be different. But if it's a few hundred or less, it's often wiser to just take it on the chin.
I hope that helps...