Does time since diagnosis play a role in benefits of exercise when studying patients with Parkinson's disease?

That is an excellent question. This was not addressed in this article. I would think that, and I think there are studies to suggest that the benefits of exercise may be more in the early to mid-stage of the disease when gait is intact.

Time since diagnosis is a relatively subjective estimate of disease onset. It is also well-known that Parkinson’s disease starts much earlier than the onset of motor symptoms would lead us to believe. We consider overall motor symptom severity to be a more important factor than time since diagnosis ...