@Mindy17 , the only thing that makes sense is that Airbnb's request to submit a CC number to facilitate a refund concerns a PAST reservation, in other words you have gotten paid for it, Airbnb only withheld their portion of the service fee, and the guest's account shows zero balance. If Airbnb has no reserve anymore in that guest's account, why would they want to advance payment to the guest from their own funds when you are the one that decided to give the guest a refund? sounds like Airbnb did that in the past but just recently changed their policy on it.
From a purely finacial angle the new policy makes sense: to subtract that amount from a future payment to you, or what you call debiting your Airbnb account, is an iffy business proposition: you might cancel your account, never do another booking, or something else might come up., and Airbnb will never be able to recover that amount from you , so businesswise it makes perfect sense that Airbnb doesn't want to do that, it's like extending an unsecured loan to you.
But if the refund concerns a future booking and you haven't gotten paid for the guest's stay yet, then this request by Airbnb to submit your CC number makes no sense, as Airbnb is holding all the $$ for that booking and hasn't released them to you yet. but I don't see why a host would go to the resolution center for that, instead of simply going to "change reservation" and adjust the $$ amount that way. Maybe going to the resolution center is what created the problem, and then the "lets get the host's CC number" routine got triggered.
your scenario is different: the guest cancelled immediately, you have not received any payment from Airbnb. the refund situation is between Airbnb and the guest, you are not involved in that at all, you do NOTHING. But apparently you tried to do something, and then also, the "give us your CC number" routine got activated. Sorry, don't do anything!