I've looked at the details Airbnb gave for the wearing of facemasks. It says "All Airbnb Experiences hosts and guests must wear protective face coverings as recommended by regulatory agencies or authorities such as the US CDC or the European CDC".
Looking on the European link it doesn't say anything about having to wear a face mask when outside. Our Airbnb is our coach house which is the other side of our driveway. We greet our guests on our side of the outside gate. We make sure we keep at least 2m away from our guests. So the way I read it, we don't need to wear masks in this scenario. Obviously, if we have to enter the Coach House for any reason while the guests are there (ie, something not working properly) we will wear a mask then.