You missed the working part. Success was a prerequisite to their after the fact feelings. At least some of the control group will be in old age but still alive when we know it woris. They might not know if it is infinite life (and side effects may turn it into die at 85, so some control may outlive the intervention group after the study ), but they will know on average they did worse

People opt into the study in the first place. I'm willing to bet that no one opts into the study hoping to be in the control group.