But the landlord is not a party to the contract between Airbnb and the host. If any damage were to occur to the building, the landlord would need to sue the host and collect from him, and Airbnb says nothing about paying for legal expenses. (And once the landlord got involved, it would probably ultimately result in eviction proceedings against the host for violating the terms of his lease.)
Note that the Airbnb guarantee doesn't insure the host's personal property: "The Host Guarantee is not insurance and should not be considered as a replacement or stand-in for homeowners or renters insurance." However, using your apartment as a rental property probably violates the terms of a standard renters insurance policy, so the insurance company would probably not pay for losses that were due to an Airbnb guest.
You're talking about a situation in which significant damage is done to the building AND the host doesn't report this to Airbnb with photos and evidence. There are bound to be horror stories with any service (hotels certainly included), but I'd expect this particular combination of all-around irresponsibility to be rare.
Note that the Airbnb guarantee doesn't insure the host's personal property: "The Host Guarantee is not insurance and should not be considered as a replacement or stand-in for homeowners or renters insurance." However, using your apartment as a rental property probably violates the terms of a standard renters insurance policy, so the insurance company would probably not pay for losses that were due to an Airbnb guest.