If low-risk groups engaged in more risky behavior and no other factor changed , you'd expect death rates of hospitalized patients to drop because a larger share of the hospitalized would be from groups at lower risk of death if hospitalized with COVID. Also, if hospitals learned more about interventions (like that ventilation wasn't appropriate as early or in as many cases as initially thought), and nothing else chan…
The article says the analysis controlled for age. Survival has improved in every age range.
Even if age was the only risk factor (in which case controlling for age would negate the first and third points), the second point would cover that. But age isn't the only risk factor, so all three points are still relevant.