It is not as simple as that though. The act does not protect against negligent practice. For example if a doctor does something that is against the standard of care that results in harm to the patient then it wont be indemnified by this act. Most doctors these days are heavily specialized so may not necessarily be up to date on areas of medicine outside their expertise so can provide care or opinion that is negligent or not standard in good faith truly thinking they are doing the right thing for the sick person.

Also the act does not indemnify against the condition of the doctor - if you have say you had three glasses of wine or took an ambien, and went to help because you were the only doctor available, then you are liable for for actions and not exempt by the act and can be sued successfully for providing (bad) medical care under influenceof drugs or alcohol. If you gave medical care outside scope of your expertise - for example you are a psychiatrist and then tried to administer a drug or treatment you have no training to do (like use a coat hanger to drain a pneumothorax as a doctor famously did years ago) then you can certainly be sued if things go wrong. Also I don't know for sure the jurisdiction issues assuming the flight originated and was diverted to a non US airport.

True there are practically no cases of doctors or nurses being successfully sued for inflight good Samaritan acts but it doesn't mean it cant happen. I don't think anyone wants to be that index case.