I can see exactly why they would think it was okay. What they were trying to do was to figure out whether twitter moods could predict the unpredictable random movements of the market, rather than the outside edges of the bell curve when anyone could have predicted what would happen, and where the market moves might even be seen as a cause of the twitter moods rather than a result, or a co-result of some other process. Basically, it's the hardest place to test the correlation.

Since they got great results, it'll be interesting to see them extend the analysis and see if any interesting dynamics show up.