If you are booked through Airbnb it sounds like the host is using a third party property management agency.
this should have been disclosed on the listing when you booked. @Mandy321 (does it not say on the listing that they are the hosts/co/hosts)
The agency can’t tell you that you need to abide by different booking conditions than the ones you booked under through Airbnb.
If they want you to cancel by 21 September get them to confirm that you will get a full refund including the Airbnb fee. They are absolutely in a position to do this. They just need to call Airbnb and check if they will do this. Or they can ask Airbnb to cancel. Do not cancel yourself until you know you will get the fees back too.
As others have said the government regulations for December haven’t been announced yet. If they decide nearer the time to extend the 6 person rule then Airbnb will extend their EC policy and you will get a full refund regardless of what this property management say.