that's insane. i can't imagine just how distressed this makes you feel tbh, but i hope you'll be able to forget about this asap and move on to much much better hosting experiences.. airbnb should know scams are a thing, but i guess it's usually the guest doing something like this after the stay to get a refund and thus, a free trip. in this situation my only guess is, he found a more affordable place or his plans changed, and that's the best he could think of to get some money back.

from your communication with him, do you know whether he's an international guest or at least from another city? because if he's from the same city as you are, with the way he's behaving, even if you do get all/some money back, i'm not sure it's worth the trouble he could try to cause. although i admit it's the paranoid way of thinking.

one thing i can suggest is see how much it would cost to invite in a specialist, have a quick check and have him provide the document that there's been no mold recently (if that kind of check is even a thing?). if mold has been airbnb's official reason for refunding him, you providing that document might work, but i'd also check in advance by calling them and asking.