Speaking as someone with a health care worker in the family who just went on strike, I think this makes a lot of sense. When hospital workers go on strike, it often forces the hospital to go on diversion, leading to other hospitals taking on all new patients, who are usually in some form of medical crisis and have a higher chance of dying than stabilized patients already under care. Thus, it makes sense that a hospital which has workers that go on strike have a lower death rate on average.