You're right that that's a possibility. A completely undiagnosed underlying condition would be missed by that type of control group. But we know that COVID adm

You're right that that's a possibility. A completely undiagnosed underlying condition would be missed by that type of control group. But we know that COVID admittances caused a big increase in the total number of people hospitalized - to the point of overwhelming some health care systems. That strongly suggests that underlying conditions would not have sent as many people to the hospital anyhow if COVID had never appeared and infected them. It is possible you explain the mechanism, though. Perhaps there are lots of people with undiagnosed underlying conditions, and getting a serious case of COVID is enough to worsen those conditions into life-threatening ones. (Perhaps this only hastens the inevitable for some of them.) If this is the mechanism then both you and the article's hypotheses would be correct.