"Dr. van Praag says she is fairly certain the drugs themselves did not cause the brain changes because of the blood-brain barrier."
"Fairly certain"? Don't we normally want statistical levels of probability for scientific results? What kind of science and statistics is this lady using? Oh, and lots of drugs cross the blood brain barrier - in fact their are a whole class of drugs that have various affects on the brain called "Smart Drugs." or nootropics. Besides all those uppers and downers there is even common caffeine's affect on brain activity.
Not saying exercise doesn't have positive impact on the brain activity - especially as it raises O2 levels (like some drugs) in the brain making higher brain metabolic rates possible, but the scientific rigors of this study are far from apparent in this article..