Would like to know the answer myself. Was the fall witnessed? Was it from an obstacle or unsafe issue? That would fall under homeowners usually but they may not cover it because of Airbnb. Be careful because back injuries can be a big way to scam.
If the fall happened in your home, your homeower's insurance should be notified and you follow their instructions. The Air BNB Host Protection is secondary to your plan. If not covered by your insurance, you can submit to Air BNB. It is best to check the wording of both your insurance and the wording of the Air BNB plan. There are exclusions in both. Good luck with this situation as it is a complicated one.