I skimmed the paper, but I couldn't find very much information on how they did the cross validation (like, what dates, they trained, and then what dates they tested the prediction) Also - I do believe that tweet sentiment can predict the stock market, but not on such a large timescale. I would guess that any analyst reading the news could have a good estimate of sentiment, at least as good as the twitter opinion finder. I think the twitter opinion finder is useful when you want to measure sentiment at a rate higher than that which humans can do it.