My guess is that they're not talking so much about the broad market as about being able to predict individual stock returns relative to the market. Having access to search-, browsing-, and click-streams of most people doing research on stocks could give them a strong indication of the realtime supply of and demand for stocks before individual and institutional investors execute transactions. My search history is probably the single best record of what I've been thinking about, and any competitor with access to it would have an excellent idea of what trades I have planned. If all Goldman Sachs traders were even stochastically telepathic, they'd have an unbeatable edge in the market.