Is there a reason you believe that the causality should be reversed? The article suggests they did tests post retirement and observed the rate of decline.
Could be stated that overworking early in life may lead to cognitive decline post retirement.
Someone with health declining over a long period will retire and withdraw from everything, and then later on be tested to find that their health has been declining I think you will find that most retired people are dead, just by virtue of the fact that all dead people retire!
The study talks of rate of subsequent cognitive decline post retirement. There's no fixed state, which once reached, people start retiring. I think the study does control for reverse causality. It is not a correlational study.