The closest thing I had comparable to this was guests who smashed windows, bled everywhere, and left $6,000 worth of damages, used a fake ID with airbnb, and disapeared into the night.
We filed a claim with airbnb, and after 3 months they reimbursed me 100%. IT takes a while, but they did cover it. I'm unsure if they'll cover your $20k damage because that's insane. I've never had a fire in a home so I truly can't say I understand how it damages the property, but I've had oven fires, and lots of wood-burning fireplaces, so I don't quite know if the smoke/ash is a big deal? I'd personally just have it professionally cleaned for $1000, and replace the furniture and let it go. You can certainly try to get $20k from airbnb, but in the meantime you're going to have to have guests there. so I'd do the $1000 cleaning + furniture now, get guests back in the property, and run as usual, while seeing what you can get out of airbnb. You kind of have to be scrappy in this business, nothing will be perfect. Or do as suggested above, and file with insurance.