@Gigi69
I know your post is a few weeks old now, but if it's any consolation, COVID-19 is no longer a part of the extenuating circumstances policy: https://www.airbnb.co.uk/help/article/1320/extenuating-circumstances-policy
Of course, it doesn't help for the cancellations you got prior to 31st May.
It seems bizarre to me how Airbnb CS has handled this situation because it's completely contradictory.
1. Guest informed Airbnb on the date of check in that someone in his/her party had COVID and they wanted to cancel. Fine. However, those guests should then never have stayed and, if they did, which you had proof of, you are entitled to at least the nights that they did stay. Even if they had to cancel during their stay due to COVID (which is obviously not the case as they reported it on day one), you should have been paid for the nights they stayed. Seeing as they requested the refund and then went on to stay there, I would think you are entitled to a full payment because they were clearly abusing the Extenuating Circumstances policy.
2. If the guests are claiming they never stayed, and Airbnb is disregarding/not believing your proof, what justification at all do they have for cancelling all the other reservations for the month for that unit? The only reason to do this is if the guest with COVID actually stayed there!
It would be great to know the outcome of your situation. It sounds really unfair and if you haven't yet had a resolution to it/received your money (personally I think you should be compensated also for the other cancelled bookings if indeed no one with COVID stayed) I would continue to badger Airbnb CS about this, quoting their own policy back to them.