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.
The article mentions controlling for this in numerous ways. It was looking at elective surgeries, as well as county wide mortality rates and not just a single hospital. That said, in the elective surgery cases it could well be a spin on what you're mentioning and people just deferred their surgeries until after the strike. You'd think surely they also controlled for this, though.
