My first reaction is that this kind of accusation won’t hold up with hosts, with Airbnb, in court, or anywhere else. It is impossible to tell where a person might have picked up a virus. This goes equally for hosts that might claim after the fact that a guest brought COVID into their house. It’s just not provable.
Have you read yet about any instances of this accusation?
I’ve wondered along the same lines how it can be proven that a host did/did not follow the COVID recommendations for cleaning. We all should follow the recommendations out of an abundance of caution, but I don’t see how proof can be established.