I think what Larkin et. al. might be seeing is patients grieving over having their life destroyed by a highly impairing illness. Over time these patients will find their balance again despite ongoing illness. So early on in the illness, patients might be an emotional wreck, but later on they will have nearly normal mental health despite still being as disabled as before.
The questionnaires used by the incompetent quacks, on the other hand, ask patients how they behave, and assume the entire cause of behavior is mental health. Don't go out and party, or enjoy reading books anymore? You're depressed. Can't go shopping by yourself, or have trouble sleeping? You're anxious.
It comes down to inappropriate research practices and equating physical and cognitive disability with a mood disorder. Larkin & Martin are either too stupid to know the difference, or they're also willing to exploit patients for their own gain.