Once the author started taking pills independently of their stress level, the variance of differences diminished a lot. I'd wager this supports the mean reversion hypothesis.
Also, while I agree with their general conclusion that theanine probably doesn't reduce stress, I'd give assign more probability to the hypothesis that theanine does work, but in other design settings. For example: drinking tea instead of taking pills, or measuring stress levels after a day instead of an hour, or evaluating the difference across time instead of in time chunks.