> - How are the patients selected for transplants? Scarce organs are probably not given to patients who have other serious impairments as well.

Even though being healthy helps move you up the list of organ transplant recipients. It would still be really strange if people who got organ transplants were healthier than the general population, who mostly do not need organ transplants.

>- How were the (non)-dementia patients assessed? No formal diagnosis of dementia != no dementia.

But it would be weird if organ transplant recipients were 10x less likely to get a diagnosis of dementia (assuming the same base rates). I would think because of their heavy interaction with the healthcare system they'd have a high rate of diagnosis.