They appear to be already accounting for that since they are measuring 30-day mortality for acute conditions. They’re saying it’s possible the reduced mortality is due to the high risk procedures actually being unnecessary.
As an argument to illustrate why 30 day mortality isnt a long enough period, imagine this scenario: you have a cancer that has a 90% chance of killing you in one year. You will be cured if you get the surgery tomorrow, but the surgery has a 30% mortality rate. In one month, 30% or those who got the surgery will die, where all those who didn't get the surgery will still be alive. In one year, 70% of those who got the surgery live, whereas only 10% who didn't get the surgery are alive.
